The Tory party co-treasurer Peter Cruddas has resigned after it was revealed he was offering access to the prime minister and chancellor for up to £250,000. He was forced out after footage emerged of him apparently making the offer to undercover reporters.

In his resignation statement last night the senior Conservative official responsible for collecting donations for the party said he deeply regretted the repercussions of his "bluster" during the recorded conversations. He added: "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. Similarly, I have never knowingly even met anyone from the Number 10 policy unit.

"But in order to make that clear beyond doubt, I have regrettably decided to resign with immediate effect."

Cruddas, the founder of online trading company Currency Management Consultants, is heard discussing how much access different-sized donations would get, during an undercover operation run by the Sunday Times.

In the footage, he is heard to say: "Two hundred grand to 250 is premier league … what you would get is, when we talk about your donations the first thing we want to do is get you at the Cameron/Osborne dinners.

"You do really pick up a lot of information and when you see the prime minister, you're seeing David Cameron, not the prime minister. But within that room everything is confidential – you can ask him practically any question you want.

"If you're unhappy about something, we will listen to you and put it into the policy committee at No 10 – we feed all feedback to the policy committee."

A Tory party source said Cruddas's position had been made untenable by the revelations. He added: "He has only been in position for three weeks but has clearly gone over the top and well beyond anything that would be tolerable to the party. It appears a case of him showing off."

The newspaper claims the offer was made even though Cruddas knew the money would come from a fund in Liechtenstein that was not eligible to make donations under electoral law. Options said to have been discussed included creating a British subsidiary or using UK employees as conduits. The overseas clients were, in fact, reporters posing as wealth fund executives who had made clear they wished to develop contacts with the prime minister and other senior ministers to further their business.

The revelations will be a considerable embarrassment for the government, especially in the wake of claims in the budget of a crackdown on wealth in offshore havens. Cameron has also made political mileage out of his intention to run a sleaze-free government.

In the last 18 months there have been three major resignations: the chief secretary to the treasury, David Laws, energy secretary Chris Huhne and defence secretary Liam Fox.

During a three-month investigation the Sunday Times claims to have hired Sarah Southern, a former Cameron aide now working as a lobbyist, who advised that making a "huge donation" was the best way to gain access to senior government figures. Her connections are said to have led to a two-hour meeting with Cruddas this month in which he laid bare the extent to which the party has been prepared to sell access to Cameron.

Last night Cruddas said he had acted without the knowledge of the leadership of the party. A Tory spokesman added: "No donation was ever accepted or even formally considered by the Conservative party. All donations to the party have to comply with requirements of electoral law, and these are strictly enforced by our compliance department."

The disclosures appear to contradict previous claims by the Conservatives that their high-value donor groups, such as the "leader's group", are for genuine supporters who do not seek to influence policy in return for their cash.

They also raise questions about the role of the prime minister. Months before taking office, Cameron warned that this type of "secret corporate lobbying" was the "next big scandal waiting to happen".

Yet the Sunday Times claims the meetings, at which Cruddas claimed "premier league" donors could lobby the prime minister directly, have not been declared to the public.

Cruddas, who built a £750m fortune through financial spread-betting, is also a member of the party's controlling board.