COMMONS Speaker Betty Boothroyd has been asked to investigate reports that ``top IRA men'' were invited to Parliament by Labour MPs - and that one of them was left unattended there.

Leader of the House Tony Newton asked her to look into claims that Sinn Fein members, who belonged to the IRA's ruling Army Council, were the guests of Labour MPs.

One of the men was allegedly allowed to go to the lavatory unaccompanied for 20 minutes, in a security lapse which breached Commons rules.

The Prime Minister yesterday attacked ``the stunning naivety'' of MPs who did not realise the connection between Sinn Fein and the IRA.

The issue was raised in the Commons by Tory Elizabeth Peacock, just as security forces confirmed that a 600lb car bomb had been defused outside police headquarters in Londonderry.

A row broke out after a newspaper report that MI5 had complained to the Labour leadership about an IRA man who was allowed to wander around the Commons on his own.

The mystery deepened when the Home Office said yesterday that MI5 was denying it had conducted a surveillance operation on the Sinn Fein politicians at the Commons.

The security service also insisted that it did not complain to the Labour leadership.

However, it is believed that Labour Chief Whip Donald Dewar did summon three MPs, including left-wingers Ken Livingstone and Jeremy Corbyn, to a meeting to discuss the alleged security breach after concerns were raised.

Shadow Home Secretary Jack Straw said: ``We take any complaints, or representations, about breaches of security very seriously indeed.''

Mr Livingstone said the Sinn Fein members he had met were not among known members of the IRA Army Council.

He poured scorn on the idea that members of the army council would come to London ``to case the joint for bomb runs''.

He said about seven Labour MPs had met Mr Mitchel McLaughlin, chairman of Sinn Fein in Northern Ireland, about three weeks ago, because Mr McLaughlin wanted to ``see what the current thinking was about the ceasefire''.

Mr Livingstone said: ``It's like the bad old days of the mid-70s with people inside MI5 living in a fantasy world of their own. No wonder the IRA keep on getting away with so much if that's the competence of MI5's detective work.

``Anybody can walk into the House of Commons, into the central lobby, which is where I met them.''