So Romania have already won the tie, but GB's Jocelyn Rae and Laura Robson will take on Romania's Simona Halep and Monica Niculescu in the doubles dead rubber.
There are smiles on court from both camps as Halep goes in search of her third victory of the weekend.
It's a jovial atmosphere inside the stadium and if it's anything like the Davis Cup, you can expect trick shots and unexpected players appearing on court.
We won't be running live text commentary on this one.
But you can watch live coverage of that by clicking on the play button at the top of the page.
Post update
So that's it from me.
A rather more enjoyable morning after yesterday's eventful day.
We were treated to some outrageous shots and brilliant tennis - the best of which will be up on the site later today.
If reports are correct, and the Watson v Begu match did end without a handshake, it's a sour way to end a controversial weekend in Constanta.
I'm sure we'll be talking about this one for a while yet.
Enjoy the rest of your sunny Sunday!
Begu's winning moment
Take a look at the moment when Begu handed Romania a 3-1 lead in the tie.
Video content
Video caption: GB lose Fed Cup tie to Romania as Watson beatenGB lose Fed Cup tie to Romania as Watson beaten
'Not a weekend we will forget'
Russell Fuller
BBC tennis correspondent in Romania
The last few games of that match were a reminder of the
drama Fed Cup and Davis Cup matches usually throw up.
It is not a weekend we
will forget in a hurry, but there is no doubt the best team won.
The zonal
competition of Euro Africa Zone 1 beckons once again for the British team in
February next year. It is a routine they are tiring of.
Sunday singles
After an eventful Saturday, Sunday was a rather more straight forward affair for Romania.
World number five Simona Halep beat GB's world number seven Johanna Konta 6-1 6-3 to put Romania 2-1 in front.
Irina-Camelia Begu then saw off Heather Watson 6-4 7-5 to hand Romania victory.
There were still issues with the crowd - who were repeatedly told by the chair umpire to quieten down during points - but GB were simply beaten by a better side.
Post update
Post update
Watson 4-6 5-7 Begu
Mel South
Former GB Fed Cup tennis player
I don't think it will be long before Britain get back to the World Group somehow or other. They have built such a strong team spirit in that group and Anne Keothavong has stepped into the role of captain superbly.
Hopefully next year they can have a slightly kinder draw.
Post update
Watson 4-6 5-7 Begu
Commentator Peter Odgers on BBC Sport coverage: Begu wins the battle for Romania as they head back into the World Group. It is not third time lucky for Great Britain, but they can be proud of how they performed in a tie they were always underdogs in.
You couldn't have asked much more from Heather Watson on Begu's favoured surface.
Post update
Watson 4-6 5-7 Begu
Heather Watson gave that absolutely everything in difficult circumstances.
There wasn't much between them - despite almost 80 ranking places separating the pair.
But it's Romania who move back into the world group, as GB's attempt to escape the zonal group comes to an end.
Game, set and match
Watson 4-6 5-7 Begu
And that's that.
Begu holds her final service game to take the match - and hand Romania an unassailable 3-1 lead in the tie.
BBC SportCopyright: BBC Sport
Match point for Begu
Watson 4-6 5-6 Begu
Watson is agonisingly close to pulling off that backhand but it's into the net and that brings up match point for Romania...
Post update
Watson 4-6 5-6 Begu
Watson finds herself 30-0 down - not a good position.
But a decent return and she's back in this game.
The Brit needs one last fight - and she might get a chance as Begu puts her first serve into the net.
And Watson is staying with Begu, she's crawled back to 30-30 as the Romanian fires a weak backhand into the net.
Post update
Watson 4-6 5-6 Begu
The Romanians are really ramping up the atmosphere in Constanta.
Music is blaring out as the players return to the court.
Begu serving for the match, and the tie.
BBC SportCopyright: BBC Sport
Begu breaks
Watson 4-6 5-6 Begu*
A third break point for Begu and this time she takes it.
Romania are just one game away from winning this tie.
Post update
Watson 4-6 5-5 Begu
The Romanian comes up with the error, but a decent return foxes Watson and she brings up a second break point.
In difficult circumstances Watson fields a good second serve to get herself out of trouble.
This is nail biting stuff.
Break point Begu
Watson 4-6 5-5 Begu
This is a game of small margins.
Watson's backhand just falls the wrong side of the line and Begu has a chance to move within one game of the match.
BBC SportCopyright: BBC Sport
Post update
Watson 4-6 5-5 Begu
Some terrific tennis being played in Constanta.
Begu pulls off a wonder shot of her own with a superb backhand down the line.
And her next shot clips the line, bouncing off in an unexpected direction, which stumps Watson.
We're at deuce once again.
Post update
Watson 4-6 5-5 Begu
What a shot!
Heather Watson returns a fierce backhand with a beautiful sliced drop shot. Begu was nowhere near that one.
And it's exactly the time to be pulling off winners like that.
Watson up 30-0 in her service game.
BBC SportCopyright: BBC Sport
Begu holds serve to level
Watson* 4-6 5-5 Begu
A 13th backhand winner for Begu, compared to just four for Watson, puts her in command of this game.
And she follows it up with a solid forehand to move within one point of taking the game.
And there you have it. A quick service game and we're back to 5-5.
Post update
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Live Reporting
Louise Gwilliam
All times stated are UK
Get involved
Video caption: GB lose Fed Cup tie to Romania as Watson beatenGB lose Fed Cup tie to Romania as Watson beaten 
BBC SportCopyright: BBC Sport 
BBC SportCopyright: BBC Sport 
BBC SportCopyright: BBC Sport 
BBC SportCopyright: BBC Sport
Latest PostDoubles to go ahead
So Romania have already won the tie, but GB's Jocelyn Rae and Laura Robson will take on Romania's Simona Halep and Monica Niculescu in the doubles dead rubber.
There are smiles on court from both camps as Halep goes in search of her third victory of the weekend.
It's a jovial atmosphere inside the stadium and if it's anything like the Davis Cup, you can expect trick shots and unexpected players appearing on court.
We won't be running live text commentary on this one.
But you can watch live coverage of that by clicking on the play button at the top of the page.
Post update
So that's it from me.
A rather more enjoyable morning after yesterday's eventful day.
We were treated to some outrageous shots and brilliant tennis - the best of which will be up on the site later today.
If reports are correct, and the Watson v Begu match did end without a handshake, it's a sour way to end a controversial weekend in Constanta.
I'm sure we'll be talking about this one for a while yet.
Enjoy the rest of your sunny Sunday!
Begu's winning moment
Take a look at the moment when Begu handed Romania a 3-1 lead in the tie.
Video content
'Not a weekend we will forget'
Russell Fuller
BBC tennis correspondent in Romania
The last few games of that match were a reminder of the drama Fed Cup and Davis Cup matches usually throw up.
It is not a weekend we will forget in a hurry, but there is no doubt the best team won.
The zonal competition of Euro Africa Zone 1 beckons once again for the British team in February next year. It is a routine they are tiring of.
Sunday singles
After an eventful Saturday, Sunday was a rather more straight forward affair for Romania.
World number five Simona Halep beat GB's world number seven Johanna Konta 6-1 6-3 to put Romania 2-1 in front.
Irina-Camelia Begu then saw off Heather Watson 6-4 7-5 to hand Romania victory.
There were still issues with the crowd - who were repeatedly told by the chair umpire to quieten down during points - but GB were simply beaten by a better side.
Post update
Post update
Watson 4-6 5-7 Begu
Mel South
Former GB Fed Cup tennis player
I don't think it will be long before Britain get back to the World Group somehow or other. They have built such a strong team spirit in that group and Anne Keothavong has stepped into the role of captain superbly.
Hopefully next year they can have a slightly kinder draw.
Post update
Watson 4-6 5-7 Begu
Commentator Peter Odgers on BBC Sport coverage: Begu wins the battle for Romania as they head back into the World Group. It is not third time lucky for Great Britain, but they can be proud of how they performed in a tie they were always underdogs in.
You couldn't have asked much more from Heather Watson on Begu's favoured surface.
Post update
Watson 4-6 5-7 Begu
Heather Watson gave that absolutely everything in difficult circumstances.
There wasn't much between them - despite almost 80 ranking places separating the pair.
But it's Romania who move back into the world group, as GB's attempt to escape the zonal group comes to an end.
Game, set and match
Watson 4-6 5-7 Begu
And that's that.
Begu holds her final service game to take the match - and hand Romania an unassailable 3-1 lead in the tie.
Match point for Begu
Watson 4-6 5-6 Begu
Watson is agonisingly close to pulling off that backhand but it's into the net and that brings up match point for Romania...
Post update
Watson 4-6 5-6 Begu
Watson finds herself 30-0 down - not a good position.
But a decent return and she's back in this game.
The Brit needs one last fight - and she might get a chance as Begu puts her first serve into the net.
And Watson is staying with Begu, she's crawled back to 30-30 as the Romanian fires a weak backhand into the net.
Post update
Watson 4-6 5-6 Begu
The Romanians are really ramping up the atmosphere in Constanta.
Music is blaring out as the players return to the court.
Begu serving for the match, and the tie.
Begu breaks
Watson 4-6 5-6 Begu*
A third break point for Begu and this time she takes it.
Romania are just one game away from winning this tie.
Post update
Watson 4-6 5-5 Begu
The Romanian comes up with the error, but a decent return foxes Watson and she brings up a second break point.
In difficult circumstances Watson fields a good second serve to get herself out of trouble.
This is nail biting stuff.
Break point Begu
Watson 4-6 5-5 Begu
This is a game of small margins.
Watson's backhand just falls the wrong side of the line and Begu has a chance to move within one game of the match.
Post update
Watson 4-6 5-5 Begu
Some terrific tennis being played in Constanta.
Begu pulls off a wonder shot of her own with a superb backhand down the line.
And her next shot clips the line, bouncing off in an unexpected direction, which stumps Watson.
We're at deuce once again.
Post update
Watson 4-6 5-5 Begu
What a shot!
Heather Watson returns a fierce backhand with a beautiful sliced drop shot. Begu was nowhere near that one.
And it's exactly the time to be pulling off winners like that.
Watson up 30-0 in her service game.
Begu holds serve to level
Watson* 4-6 5-5 Begu
A 13th backhand winner for Begu, compared to just four for Watson, puts her in command of this game.
And she follows it up with a solid forehand to move within one point of taking the game.
And there you have it. A quick service game and we're back to 5-5.
Post update
BBC Sport app users may have to click the link to view the content.