The committee is already due next week to discuss bringing Neil Hamilton and Tim Smith, who resigned his Northern Ireland ministerial post last week, within the investigation. But Mr Alton is also demanding details of allegations put to Sir Robin Butler that Mr Howard emphatically denied in a statement yesterday.
Mr Colvin, MP for Romsey and Waterside, admitted on Monday that he had failed to register a consultancy with Stategy Network International, a lobbying firm with strong ties to political interests in South Africa.
These two additional issues will be raised at next week's meeting, along with Mr Alton's objection to the continuance on the committee of Tory member Sir Peter Hordern - who is away in the Far East - because he was once a consultant to Mohamed al-Fayed's Harrods.
The committee went ahead with its first substantive hearing last night - minus its Labour members.
The Tory rump of the committee, plus Mr Alton, listened to tapes and studied transcripts of conversations between a Sunday Times reporter posing as a businessman and the backbench Tory MPs Graham Riddick and David Tredinnick, who originally prompted the 'cash-for-questions' inquiry. Those Tories who were unable to attend are due to listen to the tapes today.
The two MPs are each alleged to have been prepared to accept pounds 1,000 for tabling a parliamentary question.
The Labour members withdrew en bloc in protest at a vote last week to examine key witnesses in private.
Labour MPs have also threatened to withdraw from the Select Committee on Members' Interests, which polices breaches of the register of MPs' interests, if it attempts to hear in private evidence concerning Mr Hamilton's expenses-paid stay at the Paris Ritz.
Senior Tory MPs stood by John Major's rejection of Labour's demands for public hearings. One former committee chairman said: 'It would be trial by television.' MPs would face allegations without representation, he said.
During exchanges following Mr Major's Commons statement, Dame Jill Knight, a Tory on the privileges committee, criticised Labour members for complaining when its report would be published and then debated by the House. Mr Major agreed. But Doug Hoyle, a Labour member, said: 'The public will ask: What has this Government got to hide that it has got to meet in private?'
Mr Riddick and Mr Tredinnick will be examined in person next week. Bill Walker, another Tory MP approached by the Sunday Times, has also been requested to attend.
Labour, meanwhile, used yesterday's Opposition Day debate to accuse the Government of packing National Health Service trust boards with Tory supporters.