Chris Huhne says the law can be broken even if no donation is made
The Electoral Commission has rejected a call from the Lib Dems for it to look into whether George Osborne broke the law by allegedly soliciting donations.
Home affairs spokesman Chris Huhne urged the watchdog to investigate claims Mr Osborne solicited a £50,000 donation from a Russian billionaire.
But the Commission said it saw "no information" suggesting an offence.
Mr Osborne has denied claims he tried to seek a donation and has been backed by Tory leader David Cameron.
The Commission said on Wednesday that it would not launch an inquiry, as "soliciting a donation is not an offence".
But it was pressed further by Mr Huhne who wrote to them suggesting his interpretation of electoral laws suggested the shadow chancellor had been "sailing very close to the wind".
In his letter to Sam Younger, the Commission's chairman, Mr Huhne said there was a "need to establish quickly whether the law has been broken".
Financier Nathaniel Rothschild has alleged Mr Osborne and Tory fundraiser Andrew Feldman discussed the possibility of Oleg Deripaska making a donation to the Tories through his UK business LDV vans.
Mr Huhne said he believed "an offence does not require a donation but merely 'an arrangement... likely to facilitate donations from an impermissible donor'".
He added in his letter: "You should either open an inquiry into the events on Mr Deripaska's yacht or you should clearly set out clearly your interpretation of the law."
Mr Osborne and Mr Feldman strenuously deny soliciting a donation in Corfu in August, saying that the issue was raised by Mr Rothschild.
But the financier says he has a witness - multimillionaire hedge fund manager James Goodwin - to a conversation he and Mr Osborne had about the matter, and has let it be known that he would go to court over the matter.
During prime minister's questions on Wednesday, Mr Brown said the matter was "very serious" and he hoped it would be "investigated by the authorities".
But Tory leader David Cameron accused Mr Brown of "playing politics" over the row.
Mr Cameron said it was unclear what Mr Brown was talking about and said Mr Osborne has fully answered the allegations made about him.
Speaking in Fife, where he is campaigning ahead of the Glenrothes by-election, Mr Cameron said his shadow chancellor had explained what had happened in Corfu in the "clearest possible way" and was telling the truth.
David Cameron said George Osborne had answered all the allegations
"It comes back to the simple point. No money was asked for and no money was received. That's that."
He added: "This is one of the first financial scandals I can remember where there is no finance involved."
The Conservatives would "obviously co-operate" with any inquiry should it happen, Mr Cameron said, but he questioned the prime minister's motives for calling for one.
"It is not at all clear what the prime minister was talking about," he said, referring to his remarks at PMQs.
"I don't know whether the prime minister misspoke or was just trying to keep a story going and playing politics with it. I think he will have to answer that himself."
In the Commons on Thursday, Labour MP David Winnick also urged the Electoral Commission to open an investigation into the matter.
Business Secretary Lord Mandelson, who was then European trade commissioner, stayed on Mr Deripaska's yacht, moored off Corfu, at the same time as Mr Osborne stayed at Mr Rothschild's villa.
Meanwhile, Lib Dem MP Norman Baker has written to the parliamentary standards commissioner to clarify what hospitality MPs should declare in the register of members' interests.
Mr Baker said Mr Osborne should declare his stay at Mr Rothschild's Corfu home, even though he was on holiday and an old university friend of the financier, since Mr Rothschild potentially stood to benefit from their discussions.
The same rules should also apply to Lord Mandelson, he added.