DAVID Cameron is facing calls for an independent inquiry after the Conservatives' co-treasurer was filmed offering access to the Prime Minister in return for party donations of £250,000 a year.

With Westminster fighting to recover its reputation following the MPs' expenses scandal, evidence of influence-peddling emerged at the weekend.

Recently appointed Peter Cruddas resigned within hours of being exposed for soliciting large donations in return for meetings with ministers and the chance to influence policy.

Mr Cruddas urged what he believed to be wealth-fund executives to make donations bigger than £250,000 in return for direct contact and private dinners with senior ministers, right up to Mr Cameron himself. Such large donations would be "awesome for their business", he told them.

After being exposed by Sunday Times journalists, the party co-treasurer resigned and claimed that he had become carried away and was bragging of something that could not be delivered. However, the damage was done.

Mr Cameron insisted that was "not the way" the Conservative Party raised money and promised an internal inquiry. Labour leader Ed Miliband called for a full independent inquiry into what he called "incredibly serious" allegations.

Danny Alexander, the Prime Minister's senior Liberal Democrat Cabinet colleague, described the claims as "utterly disgraceful".

SNP MP Angus MacNeil pointed out that Mr Cruddas bankrolled the campaign against voting reform last year and had boasted about discussing the forthcoming independence referendum with Mr Cameron.

Mr Cruddas told the undercover reporters he had discussed Scottish independence with Mr Cameron and that they had jokingly described Alex Salmond as "the mad Scotsman".

Mr MacNeil, MP for Na h-Eileanan an Iar, said: "These dramatic revelations show Peter Cruddas was already lining up with Mr Cameron against the Scottish referendum. We should tell Mr Cameron to keep the Tory Party's grubby hands off Scotland's referendum."

Mr Cruddas, who became the Tories' principal treasurer only weeks ago, was secretly filmed claiming "things will open up" for anybody willing to donate £250,000 a year.

He said that "premier league" donors – those giving £250,000 a year – could lobby Mr Cameron directly and their views were "fed in" to the Downing Street policy unit. He said there was no point in "scratching around" with donations of £10,000.

He is said to have been led to believe the donations would come from Liechtenstein and would therefore be ineligible under election law.

The discussion is said to have included the creation of a British subsidiary and the possibility of using UK employees to make the donation.

Major donors are invited to private dinners and other events with Mr Cameron, Chancellor George Osborne and Foreign Secretary William Hague, he said.

Mr Cameron said: "What happened is completely unacceptable. This is not the way that we raise money in the Conservative Party – it shouldn't have happened. I will make sure there is a proper party inquiry to make sure this can't happen again."

Mr Miliband said: "These are so serious because it's about the way that policy is made. We need to know what access was paid for, if access was paid for, and what contributions were made and the interaction between the Prime Minister, the Chancellor and Conservative Party donors."

Demanding a full independent inquiry, Shadow Minister Michael Dugher wrote to Mr Cameron insisting he disclose which Tory donors had visited Downing Street, Chequers or Dorneywood and what policy representations had been made.

Sir Christopher Kelly, chairman of the Committee on Standards in Public Life, said: "It would be wrong to regard this as an isolated event. The parties need urgently to address the damage this does to confidence in the integrity of the political process."