Alan Johnson today insisted his deputy leadership campaign had “done what the law asked us to do” to check an alleged proxy donation of more than £3,000.

The Health Secretary said everything had been “above board” and there had been “absolutely no impropriety”. He had been “surprised” by Waseem Siddiqui’s reported claim that he donated on behalf of his brother.

Mr Johnson became the latest cabinet minister to be embroiled in a scandal over donors last night after he was accused of accepting cash through a proxy.

The revelations will cause severe embarrassment, coming just days after the resignation of Peter Hain, who faces a Metropolitan police inquiry into £103,000 of undeclared donations for his own failed deputy leadership bid.

It was reported last night that Harriet Harman, the winner in the deputy leadership race, will be interviewed by police investigating donations made to her campaign by a proxy for the controversial millionaire property developer David Abrahams.

Mr Siddiqui, a student, said he was asked by his brother, a local Labour party official, to write a cheque for £3,334 towards Mr Johnson’s failed bid to become deputy leader.

The Health Secretary admitted last night that the gift was one of four donations he had received that have not yet appeared on the register of the official watchdog, the Electoral Commission. There are conflicting accounts about when his campaign notified the watchdog.

Speaking from his constituency in Hull today, Mr Johnson insisted that reporting Mr Siddiqui’s gift to the Register of Member’s Interests was “not the best way to hide a donation”.

“We did what the law asked us to do. We checked that he is on the electoral roll, we checked that he is registered to vote in this country,” he said. “We also checked - although we are not required to - whether he was a member of the Labour Party, and he is a member of the Labour Party.

“We submitted his donation to the Parliamentary Register, it has been on there since May 9. We submitted it to the Electoral Commission. We noticed back end of last year it was not appearing on the Electoral Commission’s website, along with three other contributions, so we met with the Electoral Commission. They advised us to resubmit them all again, which we did.”

Mr Johnson insisted it would not have been possible for his campaign team to “chase down the details of every single individual” who contributed. “We actually had some people on our team who knew of him and his family.”

The Health Secretary said he was “very surprised” by claims that Mr Siddiqui’s £3,334 gift had come from a third party. “I’m as surprised as anybody,” he said. “But all I can say is that we have followed absolutely the procedures.”

There had been “absolute” honesty on the part of him and his team, Mr Johnson insisted. “What is the point of any dishonesty in this? We took the money that was contributed to our fund.”

Gerry Sutcliffe, the Sports minister who ran Mr Johnson’s campaign, insisted everything was “above-board” and donations were “over-registered”.

Mr Sutcliffe said Mr Siddiqui’s cheque had been registered correctly, adding: “It was only in December as we were closing down all the accounts ... that we checked it wasn’t on the Electoral Commission website. They asked us to resubmit. We said we have already submitted but we would resubmit it - and that’s where we are on that donation.”

Speaking on BBC Radio 4’s World This Weekend, Mr Sutcliffe added: “We kept copies of every donation we received - we were very clear because we sent some donations back from people that weren’t on the Electoral Register, because clearly that was against the rules.”

Asked if anyone on the team had spoken to Mr Siddiqui, he replied: “Not that I am aware of. “The donation came in the usual way. It was checked against the Electoral Register as we were supposed to. This guy was a member of the Labour Party - we saw no reason for sending it back.”

Mr Sutcliffe continued: “We did everything that was required of us. “We over-recorded. We actually put down donations that we didn’t need to do - it had to be anything over £1,000 and we actually registered those of £1,000."

Times Recommends

• Milk: do we need a dairy godmother?
• DIY: the women doing it for themselves
• First double portrait of Princes unveiled