Philip Webster's analysis | Tebbit's warning to Osborne | Drinks, dinners, differing accounts | The witness | Alice Miles: comment | Peter Riddell: briefing | Rachel Sylvester: analysis | Questions for Mandelson | Sketch | Osborne chronology and Rothschild reply | Rothschild fundraiser on hold | Hellraising for the super-rich | Animal analogies: give us yours | Poll: would you vote Tory? | Yachtgate: where Osborne and Rothschild differ

Gordon Brown called today for an official investigation into allegations that George Osborne, the Shadow Chancellor, solicited an illegal donation from a Russian oligarch while on holiday on Corfu this summer.

Mr Brown was asked about the row, which has left the Tories reeling, at Prime Minister's Questions and replied: "This is a very serious matter indeed and I hope it is investigated by the authorities."

The Times understands that no complaint has yet been made to the Electoral Commission over the allegation - and Mr Osborne has denied that any donation was either received or sought. But Mr Brown's words suggests that a Labour MP might be preparing to make a complaint that would automatically prompt an investigation.

Yachtgate: where Osborne and Rothschild differ

Five key questions remain unresolved over allegations that Conservatives solicited a donation from Russian billionaire

• A witness with impeccable Wall Street credentials

• Drinks, dinners and differing accounts: a weekend on Corfu

• George Osborne on the rack over new donation allegations

BACKGROUND

• Oleg Deripaska - oligarch at centre of storm

• Hellraising for the rich: how Osborne met Rothschild

• Osborne's chronology and Rothschild's reply

Multimedia

• Osborne and the oligarch

Related Links

• Analysis: Bringing the House down

• Yachtgate: where Osborne and Rothschild differ

• Tory grassroots: 'Osborne should resign'

Multimedia

• Poll: would you vote Tory?

• Comment Central - give us your animal analogies

The Prime Minister is the first senior Labour minister to comment on the affair, except for Lord Mandelson, whose presence in Corfu at the same time helped precipitate the row.

The issue is the talk of Westminster but was not raised at PMQs until the very end, when Dennis Skinner, the ever-combative Labour backbencher, used a question to taunt the Tories. "Will the Prime Minister give us a rock solid assurance that whatever he does in order to clear the debts of the nation's finances, that he will never ever meet a Russian billionaire in order to try to cadge the money, but will leave that to the sleazy Tory party opposite?" he asked.

Mr Brown's willingness to get involved suggests that Labour consider the row far from over - and will add to Mr Osborne's discomfort.

Earlier today, Lord Tebbit, who served as party chairman under Margaret Thatcher, pitched himself into the row with characteristic bluntness, telling Mr Osborne that "those who sleep with dogs will get fleas".

Mr Osborne has denied an allegation from his erstwhile friend Nathaniel Rothschild, made in a letter to The Times published yesterday, that he and Andrew Feldman, the Conservative Party chief executive, solicited a donation from Mr Deripaska in various meetings in Corfu this summer.

David Cameron's close political ally yesterday gave a detailed account of his meetings with the oligarch on the Greek island and said: "We didn't ask for any money. We didn't receive any."

But Mr Rothschild, apparently stung by Mr Osborne's earlier indiscretions about a meeting with Peter, now Lord, Mandelson on the same holiday, stood by his claims and last night named a witness to the discussions.

Tory party officials said today that neither Mr Osborne nor Mr Feldman planned to sue over the allegations, to avoid a "messy" court case in the run-up to the next election. "They know politics is a controversial business and they are pretty relaxed about it all," one said.

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