A senior Tory party member resigned Sunday after he reportedly offered access to the PM for donations of £200,000 - 250,000.
The Sunday Times released secretly recorded video footage of what it said was the centre-right party's co-treasurer Peter Cruddas making the offer to undercover reporters.
In a statement shortly after the allegations were published, Cruddas said his comments to the reporters were "bluster" but added he had resigned.
"I deeply regret any impression of impropriety arising from my bluster in that conversation," he said.
"Clearly there is no question of donors being able to influence policy or gain undue access to politicians.
"Specifically, it was categorically not the case that I could offer, or that David Cameron would consider, any access as a result of a donation.
"But in order to make that clear beyond doubt, I have regrettably decided to resign with immediate effect."
In the footage, Cruddas is seen discussing different sized donations and saying that £200,000 to £250,000 is "premier league", granting access to Cameron and his Chancellor of the Exchequer George Osborne.
"When we talk about your donations the first thing we want to do is get you out to Cameron and Osborne dinners," he says in the recording.
"You do really pick up a lot of information. And when you see the prime minister, you're seeing David Cameron you're not seeing the prime minister....
"But within that room everything's confidential and you will be able to ask him practically any questions that you want."
In his resignation statement, Cruddas said he had only recently taken up the post of party treasurer and had arranged the meeting in which he was filmed without consulting any Conservative politicians or officials.
A Conservative Party spokesman had earlier said that "no donation was ever accepted or even formally considered by the Conservative Party."
But Labour MP Michael Dugher, in comments to the media, called for a statement from the prime minister.
"David Cameron should come clean. Will the PM say exactly what he knew and when about an apparent effort to sell access and influence in Downing Street?"
In 2010, Cameron promised curbs on Britain's lobbying industry if he came to power, saying that lobbying was the "next big political scandal waiting to happen."
Source: AFP European Edition