Police here believe that the car bomb that killed a leading member of Parliament just outside the House of Commons last week was a particularly sophisticated device planted by an ultraradical Marxist faction that broke away from the Irish Republican Army.

Pending results of laboratory tests on fragments of the bomb, investigators have put credence in the claim by the Irish National Liberation Army (INLA) that it was responsible for the assassination Friday of the Conservative Party's parliamentary spokesman for Northern Ireland, Airey Neave.

In repeated telephone calls to news organizations, persons claiming to speak for the group have provided significant details about the operation of the bomb which was believed to have been attached under the driver's side of Neave's Vauxhall sedan while it was parked outside his home near Parliament's Westminster Palace Thursday night or early Friday morning.

The INLA calls itself the military wing of the Irish Republican Socialist Party, which broke away from the IRA four years ago. The Marxist splinter group has a few hundred members who want to create a communist Ireland. It has since carried on a bloody feud with the Provisional IRA for supremcy among the militants of the Catholic nationalist movement in Ulster.

The sophisticated two-stage compact bomb used to kill Neave allows the person who plants it plenty of time to flee, perhaps even out of the country, before it explodes. It had not been used before in Britain. The INLA, however, had claimed responsilibity for a similar bomb that killed a member of the Protestant Ulster Defense Regiment in Northern Ireland on March 7.

Police are concerned that this technology, already used elsewehre by Palestinian terrorists and Italy's Red Brigades, will escalate the level of violence by IRA terrorists and their allies who are trying to drive Britain out of Northern Ireland.

Large bombs believed to have been placed by the Provisional IRA exploded in the centers of two more Northern Ireland towns yesterday and today, injuring seven people, including two policemen. The bombs, hidden in parked cars, seriously damaged the commercial areas of both towns near the Irish border.

A new wave of IRA violence began last month with widespread bombings of Ulster cities, towns and villages and the murder of the British ambassador in The Hague. The IRA has vowed to disrupt the British election campaign with more violence.

In Parliament yesterday, new security measures were announced to protect members of Parliament inside Westminster and while campaigning for the May 3 national election.