Labour Shadow minister Michael Dugher is calling on David Cameron to launch a full inquiry over claims by ex-Tory party co-Treasurer Peter Cruddas, who has been caught offering access to the heart of government in exchange for large donations.

The PM has already said there will be an internal party inquiry - this doesn't appear to be satisfying Labour, who say in the letter:

Dear Prime Minister,

I write to you following reports in the Sunday Times today of rich individuals being offered access to, and influence upon, the heart of your Government in return for sizeable cash donations.

Suggestions that in return for large donations individuals can be granted direct access to you and the opportunity to influence policy via submissions to your Number 10 Policy Unit are incredibly serious.

When ordinary doctors and nurses have had their views so completely ignored on the issue of the disastrous NHS reorganisation, it will be all the more frustrating to hear that someone who could afford a quarter of a million pounds seems able to ensure their voice is listened too.

This is not about Peter Cruddas or any individual. It is about the way your Government is conducted.

In Opposition you said that "sunshine is the best disinfectant". I agree. It is vital that we now have full transparency in this matter. Therefore I would ask you for immediate clarity on the following issues:

· Which donors to the Conservative Party have visited No10 Downing Street, Chequers or Dorneywood since May 2010?

· Which donors to the Conservative Party have made submissions to the Number 10 Policy Unit, both orally and written, when were these made, and on what topics?

· In particular, in light of this week's Budget, will you provide details of all donors who have made representations, both written and orally, on changes to the 50p tax rate?

· In the light of these allegations can you explain why - as reported in the Independent newspaper on 22 October 2011 - you personally intervened in the Sir Christopher Kelly's investigation into party funding to request that the Committee on Standards in Public Life revise draft proposals to introduce a cap on large donations - a proposal which if implemented would have prevented such allegations of large cash donations for access ever arising?

On 8 February 2010, you said that lobbying was "the next big scandal waiting to happen. It's an issue that crosses party lines and has tainted our politics for too long, an issue that exposes the far-too-cosy relationship between politics, government, business and money." For all those words action by your Government to address this is long overdue.

Today you said that you would ensure there was "a proper party inquiry" into these matters. However, given the seriousness of the allegations about how Government is conducted, it is not appropriate for the Conservative Party to investigate itself. We need a full, independent inquiry. Therefore I ask that you now request the Independent Adviser on Standards in Public Life to launch an inquiry into this matter, to answer these and any other related questions he sees fit.

I look forward to your response.

Yours sincerely,

Michael Dugher MP

PETER CRUDDAS RESIGNATION - FULL COVERAGE

• "Things Will Open Up For You," promises former Tory co-Treasurer
• Profile - Who is Peter Cruddas?
• Analysis - Has "the next big scandal" broken?