MR John Major yesterday attacked "the stunning naively" of MPs who did not realise the connection between Sinn Fein and the IRA, as he was pressed to investigate a Commons meeting between Labour politicians and Sinn Fein representatives.
At Commons question time, Mr Major disclosed that the Leader of the House, Mr Tony Newton, had written to the Speaker, Ms Betty Boothroyd, about the incident.
Earlier, the Labour left winger, Mr Ken Livingstone, had accused MI5 of living in a "fantasy world" after reports that the security service had complained that he breached security by leaving the Sinn Fein members unattended during the visit.
Mr Livingstone was one of seven Labour politicians who met them at Westminster for talks about Northern Ireland.
But the Home Office said later MI5 was letting it be known it did not conduct surveillance at the Commons and did not complain to the Labour Party leadership.
The issue was raised in the Commons by a Tory, Ms Elizabeth Peacock, as it was confirmed that a 600lb car bomb had been defused outside the RUC's headquarters in Derry.
Ms Peacock raised a press report that three members of the IRA "Army Council" had been brought to the Commons by Labour MPs on Wednesday.
To Tory cries of "Disgraceful!", she added: "One of them was allowed to wander around for 20 minutes unaccompanied. She urged the Prime Minister: "Will you please instigate an immediate investigation?"
Mr Major replied: "I understand that the Leader of the House has written to Madam Speaker about this particular incident.
He added: "My understanding is that these representatives of Sinn Fein were invited to the House to meet a number of Honourable Members, that no prior notification about the meeting was given and that they were from time to time left unattended."
To prolonged Tory cheers, the Prime Minister declared: "I don't know what the outcome of this inquiry will be - but I think it is stunning naivety of any Honourable Member not to realise the connection between Sinn Fein and the IRA."