Westminster watchdogs rejected a fresh call for an inquiry into claims that shadow chancellor George Osborne sought an illegal donation from a Russian billionaire.
Liberal Democrat home affairs spokesman Chris Huhne wrote to Electoral Commission chairman Sam Younger urging him to launch a probe or produce a clarification of the law to explain why the allegations against Mr Osborne fall outside his remit.
But the Commission responded that it saw no reason to change its view that soliciting a donation which was not eventually made could not constitute a breach of the law.
Conservative leader David Cameron repeated his defence of his embattled shadow chancellor, insisting Mr Osborne and Tory fundraiser Andrew Feldman did nothing wrong in meeting Oleg Deripaska on board the aluminium tycoon's yacht off Corfu this summer.
But Mr Huhne said: "The law suggests that George Osborne and Andrew Feldman have been sailing very close to the wind. The Electoral Commission should either launch an inquiry or clarify its interpretation of the law."
Under the Political Parties Elections and Referendums Act 2000 it would have been illegal for Mr Deripaska, who is reputed to be Russia's richest man, to donate to a British party as he is not on the UK electoral register.
It would also have been illegal for the Tories to accept a "disguised" gift through one of the oligarch's British companies.
The Tories have insisted that, as no money was received, the law was not breached.
Campaigning for the Glenrothes by-election, Mr Cameron said: "No money was asked for and none was received. You can't really have a financial scandal without any finance. That's all there really is to say about it."
He dismissed Prime Minister Gordon Brown's call for "the authorities" to investigate the affair as a "desperate" attempt to keep the story alive.