The call by David Alton, the committee's sole Liberal Democrat, could bring Michael Howard, the Home Secretary, and the former ministers Neil Hamilton and Tim Smith within the investigation into payments to MPs, alongside Tory backbenchers Graham Riddick and David Tredinnick, who sparked the cash-for-questions controversy, and Michael Colvin.
Jonathan Aitken, the Chief Secretary to the Treasury, who revealed last night that he had been interviewed by Sir Robin, may also be included.
There is still a question mark over whether any more ministers were seen during Sir Robin's private inquiry. A priority written question from Alex Carlile, the Liberal Democrats' health spokesman, seeking the total number investigated received a non-committal answer from the Prime Minister last night.
Jack Cunningham, Labour's trade and industry spokesman, yesterday called on John Major to publish the name of the informant mentioned in Sir Robin's report, and the substance of the additional allegations against Neil Hamilton which Mr Major said tipped the balance against him. Mr Cunningham said: 'How can the privileges committee inquire into unknown allegations?
'After the Pergau Dam and Scott inquiries it is imperative that members of the House of Commons and the general public are told the whole truth and not just presented with a parody of open government.'
Number 10 appeared to accept that the committee could not investigate the allegations while their substance remained a mystery, conceding: 'There is a hole there.'
Paddy Ashdown, the Liberal Democrat leader, warned Mr Major that the row would not subside until full public explanations were given. 'If he believes the Commons or the wider public are going to be satisfied with hints about allegations made in secret he is mistaken.'
In the meantime, scepticism about the scope of Lord Nolan's committee on standards in public life was hardening. The standing committee was a 'smokescreen' because it would not deal with current allegations, said Bill Michie, Labour MP for Heeley, a member of the Select Committee on Members' Interests.
MPs have been eagerly registering interests and hospitality since the controversy broke. One rushed off to declare a visit to a racecourse at an oil company's expense.