Gordon Brown calls for an investigation into the claims
Gordon Brown has called for an investigation into claims George Osborne tried to solicit a £50,000 donation from Oleg Deripaska.
The Tories firmly deny the allegations against the shadow chancellor.
Party leader David Cameron said Mr Osborne was "right" not to ask for money from the Russian billionaire.
But speaking at prime minister's questions, Mr Brown said: "This is a very serious matter indeed and I hope it is investigated by the authorities."
Pressed after prime minister's questions about which allegations Mr Brown had been referring to in his comments, Downing Street said those made "over the past 24 hours" but would give no more details.
I am not sure which authorities Gordon thought he was talking about - the only one I can think of is the Conservative Party
Tony Wright Labour MP
The No 10 spokesman also refused to be drawn on which "authorities" would carry out an investigation, saying "whichever authorities are appropriate".
The Electoral Commission told BBC Radio 4's The World at One it would not be investigating the claims as "soliciting a donation is not an offence".
Labour MP Tony Wright, chairman of the public administration select committee, also dismissed calls for a probe.
"Gordon obviously thinks it was a serious matter, and I suppose in a sense it is a serious matter," he said.
"But we are not talking about corruption here, there was no corruption. We're not talking about law-breaking, there is no law-breaking. What there is is a massive misjudgement."
Conservative MP John Redwood said Lord Mandelson should be forced to answer questions about his presence on Mr Deripaska's yacht and he rejected claims Mr Osborne had shown poor judgement.
"There is no crime committed and this is just an absurd media fantasy," he told World at One.
But former Conservative Party chairman Lord Tebbit told the BBC that if people "sleep with dogs they get fleas" but added "I don't think George Osborne would be daft enough to ask for money".
A spokeswoman for the Parliamentary Standards Commissioner said he had not received any complaints about Mr Osborne. A complaint is required before the watchdog can look into the conduct of an MP.
Mr Osborne has emphatically denied seeking a donation from Mr Deripaska while staying with long-time friend, the financier Nat Rothschild, in Corfu in August.
Mr Rothschild says the shadow chancellor did discuss a donation in front of a witness at his home.
Mr Osborne has also denied claims that he asked about ways to get round a ban on overseas donations to UK parties.
Speaking outside his home on Wednesday morning, Conservative leader Mr Cameron said: "What matters is what you do and what you say and George Osborne was right not to ask for any money. The Conservative Party was right not to accept any money.
"And I think we have been right to set out in full all the meetings and everything that took place and others should do the same."
Mr Osborne said on Tuesday he had not sought a donation, saying that the issue was raised by Mr Rothschild.
He said: "People make suggestions all the time about donations to the Conservative Party, but we are always clear and rigorous [in assessing these]."
The Lib Dems said a "lot more questions" had to be answered about the affair and called for a parliamentary inquiry.
Mr Brown repeatedly alluded to Mr Osborne's "judgement" during prime minister's questions session but it was left to veteran left-winger Dennis Skinner, a frequent critic of Mr Osborne, to raise the issue head-on.
'Put to rest'
As the half-hour session came to an end, he urged the PM to assure him he would never "cadge money" from Russian billionaires .
It was in response to that question that Mr Brown said he hoped to see an investigation.
Labour MP Denis MacShane has written to Mr Osborne, asking him to clarify what happened in Corfu.
He said that this could allow "the matter to be put to rest".
But a Conservative spokesman said this and the prime minister's comments were a "desperate attempt to keep the story going".
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