Claims that Tessa Jowell sacrificed her marriage to save her career amid a row over her husband's finances are "grotesque", a cabinet minister says.
Ms Jowell is said to be returning to the House of Commons on Monday
Local government minister David Miliband branded "rubbish" suggestions the separation was staged by Downing Street adviser Alistair Campbell.
He told the BBC's Sunday AM that Ms Jowell and David Mills were a "devoted couple" who put their family first.
Mr Mills' lawyer blamed the split on a row over a £344,000 Italian payment.
Mr Miliband said: "The best and biggest test for any cabinet minister is whether they are good at their job.
"And what has been remarkable about the last three or four weeks is that no one at any stage has said anything but that she is doing an outstanding job.
"Anyone who knows Tessa Jowell and her husband will know that they are a devoted couple with a family anyone would be proud of.
"One can only imagine the anguish they have been going through that has led them to the decision they took."
Asked by Andrew Marr during the interview whether Alistair Campbell had been involved in the decision by the couple to separate Mr Miliband said: "I think that is a grotesque suggestion, frankly."
Mr Miliband condemned suggestions of Alastair Campbell's involvement
He rejected suggestions that Ms Jowell should be forced to quit the Cabinet.
"She has got a job to do and as long as she is doing it to a high standard I think she should carry on with it," he said.
Ms Jowell was also backed by Hilary Benn, international development secretary, who said she could "undoubtedly" hold on to her job.
Asked on ITV1's Jonathan Dimbleby programme if the separation was an attempt to save her career, he said such a suggestion was "beneath contempt".
"I don't believe that for a second. This is absolutely for real," Mr Benn said.
Labour Peer Baroness Jay, a close friend of the couple, said Ms Jowell would continue on with work as normal.
She told Sky News: "I think she is determined, because she is a very professional, very strong person, to appear in the House of Commons tomorrow and take her ministerial question time.
"I think, as usual, she will do the job she is doing extremely well."
Conservative policy director Oliver Letwin said he was "very sorry for Tessa" but vowed to go on asking questions about the affair.
"We don't know the facts of the case. What we have done instead is ask the proper questions," he told Sky News' Sunday Live programme.
He said there were questions still to be answered about the Home Office's role in attempts to extradite Mr Mills to Italy.
"It's not clear why the Home Office sent the information back that was requested to the Embassy rather than the public prosecutor in Italy.
"Those sorts of questions we will go on asking, because among other things the prime minister said in response to David Cameron that he would be looking at those and we still need to know what he has concluded."
Earlier in the week, Ms Jowell, 58, was cleared of breaching the ministers' code of conduct - because a report said her husband had not told her about a £344,000 gift he had received.
Italian prosecutors have been examining claims the £344,000 was paid to Mr Mills, an international lawyer, in return for helpful testimony in a corruption probe concerning Italian prime minister Silvio Berlusconi in 1997.
Both Mr Mills, 61, and Ms Jowell have denied any wrongdoing.
During his BBC interview Mr Miliband also revealed that on Tuesday "a monumental study of English cities" would be published outlining the issues facing the 56 largest urban areas in the country.
He said the report sets out a policy agenda on the future development of England's urban spaces.
Asked to his opinion on being seen a future leader of the Labour Party the 39-year-old said: "I don't take too much of the gossip too seriously."
He backed Gordon Brown to take over the leadership when the prime minister stands down.