Conservative Party co-treasurer Peter Cruddas offered access to the prime minister and chancellor for £250,000, the Sunday Times has alleged.

It has footage of him apparently making the offer to undercover reporters.

London-based Mr Cruddas was appointed Tory co-treasurer in June 2011 and is the founder of online trading company Currency Management Consultants Ltd.

The Conservative Party said it would investigate but pointed out no donation had actually been accepted.

Labour has challenged the Prime Minister to "come clean" about what he knew about the matter, and when he knew of it.

Will the PM say exactly what he knew and when about an apparent effort to sell access and influence in Downing Street?" Michael Dugher, Labour MP

In the footage, Mr Cruddas is heard discussing what access different size donations would get.

"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," he says.

"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 number 10 - we feed all feedback to the policy committee."

Labour questions

A statement from the Tories said: "No donation was ever accepted or even formally considered by the Conservative Party.

"All donations to the Conservative Party have to comply with requirements of electoral law, and these are strictly enforced by our compliance department."

The BBC was unable to contact Mr Cruddas via the Conservative Party.

Labour MP Michael Dugher said: "Time and again the Tory party has been the obstacle to capping donations from wealthy individuals. Now it appears obvious why.

"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?"