Labour is demanding an independent inquiry into a Tory treasurer's boast that large donations to the party could secure access to the prime minister.

Peter Cruddas quit as Tory co-treasurer after his claims, filmed by undercover Sunday Times reporters, were published.

David Cameron has pledged to hold a party inquiry into the claims, which he described as "completely unacceptable".

But Labour leader Ed Miliband said that that was not good enough and a "proper independent investigation" was needed.

Mr Miliband said it would be right for the prime minister to make a statement to Parliament on the issue.

Mr Cruddas was secretly filmed saying that a donation of £250,000 gave "premier league" access to party leaders, including private dinners with Mr Cameron and Chancellor George Osborne, and with any feedback on policy shared with Downing Street staff.

Analysis Party funding and lobbying can be two of the most toxic areas of modern politics. But when they combine (as in the case of the Cruddas episode) the results can be even more poisonous. That's why Cruddas resigned so quickly and politicians on all sides have issued swift and strong condemnation. David Cameron is stamping on any suggestion that the party's rich friends in finance can buy influence or shape policy. But it'll be hard to draw a line under this affair. Labour says questions remain. How many donors have been invited to cosy dinners or drinks in Downing Street and Chequers? Did any of them happen to mention their views on big policy decisions - like the reduction in the 50p tax rate? A trawl begins today for any evidence that donors shaped policy, and the can of worms that is party funding reform will be opened once again.

He was heard initially saying that it was not possible to buy access to the prime minister.

But he then went on to discuss what access different size donations would get.

He was speaking to the reporters posing as staff from a fake wealth fund based in Liechtenstein who were interested in doing business in the UK.

He told them: "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 said they would be able to ask Mr Cameron "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."

Mr Miliband told the BBC that the claims were "very disturbing".

"These allegations can't be swept under the carpet - there needs to be a proper independent investigation into what influence was sought, what influence was gained and what impact it had," he said.

"We need to know what happened, who paid money, what interaction there was between the prime minister and the chancellor and the people who paid money."

Mr Miliband said the revelations were serious for Britain's democracy "because people expect policy to be made on the right basis not because people have made contributions".

"I think people are bound to ask questions about whether policy is being made in the national interest or in relation to the Conservative Party interest," he said.

Mr Cameron said the incident should not have taken place.

"This is not the way we raise money in the Conservative Party. It shouldn't have happened.

"It's quite right that Peter Cruddas has resigned. I will make sure there is a proper party inquiry to make sure this can't happen again."

'Impression of impropriety'

Image caption Mr Cruddas apologised for "any impression of impropriety arising from my bluster"

In his resignation statement, Mr Cruddas said: "I deeply regret any impression of impropriety arising from my bluster in that conversation.

"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."

The Conservative Party currently has several levels of donation, with the top one being the Leader's Group,where for an annual donation of £50,000 donors can be invited to join Mr Cameronand other senior figures from the Conservative Party at dinners, post-Prime Minister's Questions lunches, drinks receptions, election result events and important campaign launches.

Transparency is, the prime minister once said, the best disinfectant. Cash for Cameron

Liberal Democrat Chief Secretary to the Treasury Danny Alexander said "reform" of funding system was necessary.

He said there was a "perception that people who make large donations - be they wealthy people from the city or trade unions - have influence. They should not have that influence, nor the perception of that influence."

Cabinet Office Minister Francis Maude said the Conservative Party would continue to make clear to donors that there was "no question of people being able to influence policy through making donations".

Mr Cruddas had been involved in fundraising for the Conservative Party since June last year, and took over as the party's principal fundraiser earlier this month.

Lord Fink will now return as principal treasurer, the party announced on Sunday morning, with Michael Farmer acting as co-treasurer.