The Tory Treasury Shadow, David Heathcoat-Amory, has written to Betty Boothroyd asking her to decide whether the paymaster general broke rules of parliamentary privilege and misled the House.

Mr Heathcoat-Amory's request follows Mr Robinson's recent forced apology to the Commons.

He told MPs: "No attempt was made by me at any time to use my position in this House to advance any commercial interests."

But Mr Heathcoat-Amory claimed there was a difference between what Mr Robinson said to the House and a letter he faxed to the late media tycoon Robert Maxwell, his then business associate, in 1990, while still a backbencher.

Mr Heathcoat-Amory said: "It has since transpired that there is a faxed letter from him indicating he lobbied the Overseas Development Administration for business on behalf of two Maxwell companies."

If the speaker decides there is a case to answer the Commons can then vote over whether to send the matter to the Committee on Standards and Privileges for investigation.

Last month, Mr Robinson was ordered by the committee to make an apology to the Commons for failing to declare various directorships in the Register of Members' Interests.

Mr Robinson, who retains the confidence of the prime minister, then made only the briefest of statements.

The government was hoping that would be an end to the matter, but the Tories are determined to pursue the minister.