Now Health Secretary Alan Johnson faces gifts probe - over £3,000 from neighbour

And Mr Johnson was forced to respond to questions over whether or not the wealthy entrepreneur gave the money as a third party acting on behalf of others who wished to remain anonymous.

The new row came only three days after Work and Pensions Secretary Peter Hain was forced to resign for failing to declare more than £100,000 worth of donations to his Labour deputy leadership campaign.

Mr Hain's career was ruined after it emerged that some of his supporters used a shadowy think-tank to channel money to him anonymously, and the Electoral Commission called in the police to investigate.

Now Mr Johnson, who was beaten by a tiny margin in the deputy leadership contest by Harriet Harman, is under pressure to reveal more details about his own funding.

The dispute centres on a £3,000 gift by Labour supporter Waseem Siddiqui, who lives near Mr Johnson's South London home.

Such donations are meant to be registered with the Electoral Commission, which then publishes them on its website. But Mr Johnson's list of donors on the website does not include Mr Siddiqui.

And in a brief statement Mr Siddiqui declined to state categorically that his donation consisted entirely of his own money.

"As a supporter and member of the Labour Party I was asked by family members who were supporting Alan Johnson if I might contribute to the campaign," said Mr Siddiqui.

"I was pleased to be able to contribute as I thought Alan Johnson was an excellent candidate."

Mr Johnson's spokesman also issued a statement which did not rule out the possibility that the money came from someone other than Mr Siddiqui.

"We had no reason to believe the donation came from anyone other than Mr Siddiqui," said the spokesman.

"We checked he was a member of the Labour Party and was on the Electoral Register and we registered the donation with the Labour Party, the Register of Members' Interests and the Electoral Commission."

A spokeswoman for the Commission confirmed that Mr Siddiqui's donation had been registered with it but had not yet been published on its website.

She could not explain why the process had taken so long, given that the deputy leadership contest ended more than six months ago.

She said that the donation was being looked at under the normal "compliance process" which involved routine checks on its "permissibility".

She could not say how long ago the Commission received details of the Siddiqui money or how long such donations normally took to process but added that "the law required donations had be registered with the Commission within 30 days of their acceptance".

Mr Johnson's spokesman said: "The money was registered with the Commission but for some reason does not appear on its website."

He could not say when the donation was registered.

Although the money does not appear on the website, it is listed in the House of Commons register of MPs' interests, which confirms Mr Johnson received a "personal donation" from "Croydon resident Waseem Siddiqui".

The money was registered on May 9 last year.

Former postman Mr Johnson's leadership campaign was supported by several prominent Labour figures including new Work and Pensions Secretary James Purnell.