Harriet Harman was under growing pressure in the sleaze row engulfing Labour last night as she was forced to pay back £5,000 given to her by a property developer who has secretly bankrolled her party to the tune of £600,000.

Ms Harman had taken the money to help to pay off the debts from her successful Labour deputy leadership campaign. But yesterday she looked vulnerable after it emerged that Hilary Benn, a fellow candidate, had rejected similar offers from a “middleman” acting for David Abrahams – as had Gordon Brown’s leadership campaign team.

Ms Harman was in further difficulties after she refused repeatedly to say if she had in fact solicited the money from the intermediary, Janet Kidd, rather than having had it offered to her. The Times has learnt from other donors to Ms Harman’s campaign that her team actively sought funding from them because they were on a list of people giving to Labour.

The Prime Minister faces further embarrassing questions about the conduct of his party when he attends the Commons today. He has already said that Labour is to pay back the £600,000 from Mr Abrahams given through middlemen because it had not been “lawfully declared”.

Last night it was revealed that one of Mr Brown’s senior officials wrote a personal letter to Mr Abrahams thanking him for his “help and support” for the party over the years. The letter from Jon Mendelsohn, Mr Brown’s chief election fundraiser, suggests that senior members of the party knew about Mr Abrahams’ contributions. There was more da mage yesterday as a fourth intermediary was named as a Labour donor. However, Janet Dunn denied knowledge of the £25,000 donation in her name and said that she had not knowingly written a cheque to the party.

The denials raised the question of whether more serious offences had been committed if Mrs Dunn was falsely portrayed as a Labour donor. A sense of farce infected the scandal as she and her husband revealed that they were, in fact, Tory supporters.

Ms Harman said of Mrs Kidd: “I do not know whether my campaign team contacted her or she was contacted by campaign team.” But Labour and Tory politicians were asking why Ms Harman, who is married to the party’s treasurer, Jack Dromey, did not raise questions about the source of the money – in particular as her rival Mr Benn was told by his supporters.

At his Downing Street press conference Mr Brown failed to give the deputy leader his unqualified backing. It was only after repeated questioning that he accepted that he “had confidence” in her. Officials later contacted reporters to say that his support for Ms Harman was “unambiguous”.

Mr Brown took the general issue of the donations head on by announcing that Labour was paying back the £600,000, and said that the way the payments had been made had been “completely unacceptable”. But he insisted he had known nothing about the arrangements until they were about to break in a Sunday newspaper.

Paying back the money will do nothing to help Labour’s troubled finances: the party faces debts of more than £20 million, including £14 million in outstanding loans.

The Prime Minister said he was appointing a retired judge and a former bishop to advise the party on changes to procedures, one of five reviews into the affair. But the pressure intensified with the disclosure that the Crown Prosecution Service (CPS) had been in contact with the Electoral Commission, which is investigating the affair. It would be up to the CPS to decide whether to bring criminal charges if the commission concluded that the law had been broken.

Mr Brown’s comments marked a rapid U-turn after Diane Hayter, the chairman of Labour’s ruling National Executive Committee, had said only hours earlier that there was no need for Labour to repay the donations. “The money was given completely legally – these donors were completely entitled to give to us, and there was nothing illegal about the donations.”

Mr Brown has acknowledged that he may have met Mr Abrahams, but he said that he had no recollection of having ever discussed funding matters with him.

Chris Grayling, the Shadow Work and Pensions Secretary, said Mr Brown appeared to have “hung his own deputy out to dry”.

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