A senior member of Parliament who served as a close adviser to opposition Conservative Party leader Margaret Thatcher was assassinated this afternoon by a bomb in his car just outside the House of Commons.

Airey Neave, 63, who was also a war hero and the Conservative Party spokesman on Northern Ireland, was killed as he drove up the exit ramp of the House of Commons underground garage inside Parliament's Westminster Palace in the heart of London.

Two terrorist factions of the Irish Republican Army immediately claimed responsibility for the death of Neave, who was an outspoken supporter of strong security measures in Ulster. He would have become Northern Ireland secretary, overseeing British rule in Ulster, should Thatcher become prime minister after Britain's May 3 national election.

Neave's murder is the first assassination of a member of Parliament since the shooting of Spencer Perceval, who was then prime minister, in the lobby of the House of Commons in 1812.

The IRA recently vowed to strike inside England during the election campaign as part of a new wave of terrorism aimed at forcing Britain out of Northern Ireland. Last week, commercial areas were bombed in cities and towns throughout Ulster and the British ambassador to the Netherlands was murdered by gunmen outside his residence in The Hague.

Neave was the most prominent British public figure to be assassinated in London since the IRA killed Field Marshal Sir Henry Wilson here in 1921. The IRA tried unsuccessfully to kill other members of Parliament with car bombs in several incidents in 1974 and 1975.

The half-pound bomb that blew up from underneath the driver's side as Neave drove out of the concealed garage today was heard as a muffled boom inside the Commons chamber, where a debate was in progress.

Windows shook and the explosion's shock could be felt throughout the parliamentary buildings around the courtyard over the garage. Dark smoke rose from the sunken ramp up the outside of Westminister Tower, in which Big Ben had just struck 3 p.m.

Members of Parliament, journalists and police officers rushed out to find Neave's blue Vauxhall sedan halfway up the narrow concrete ramp, surrounded by debris from the blast.

British Press Association reporter David Healy said Neave "was still in his seat, almost standing, his face bloody and blackened. . . He was unrecognizable."

Neave's legs had been blown off by the blast. He was trapped in the wreckage for half an hour, while a doctor and nurse worked to save him, and died shortly after being extricated and rushed to a nearby hospital.

Officials at Scotland Yard, who had already met with aides of leading politicians about the campaign, began reviewing security plans.

Prime Minister James Callaghan said "no effort will be spared to bring [Neave's] murderers to justice and to rid the United Kingdom of the scourge of terrorism. This abhorrent act has robbed our country of a distinguished public figure and a very brave man."

Ireland's prime minister, Jack Lynch, also promised his support for any joint action necessary to catch the killers and stop IRA terrorists, whom he called "the common enemy."

"The IRA has again demonstrated its contempt for human life and values, and for the reputation of the Irish people," Lynch said in Dublin.

Catholic and Protestant political and church leaders in Northern Ireland also joined in their condemnation of Neave's murder. Gerry Fitt, the Catholic Social Democratic and Labor Party member of Parliament from Ulster, said Neave had only recently promised him to look into allegations of mistreatment of IRA terrorist suspects by Ulster police. The IRA seeks union of Ulster with the Irish Republic.

Thatcher, who has been a friend of Neave since they studied law together, said "the assassination" has left his friends and colleagues as stunned and grief-stricken as his family. He was one of freedom's warriors, courageous, staunch, true. He was very gentle, kind and loyal. He lived for his beliefs and now he has died for them."

Neave, who would have been a key figure in Thatcher's campaign to become prime minister," managed her rise to leadership four years ago.

Neave, born into an upper-class family attended Eton and Oxford. During service with the Royal Artillery in World War II. He was wounded and taken prisoner in the battle for Calais in 1940.

Disguised as a German soldier, he became the first British officer to escape from the Nazis' reputedly escaped-proof Colditz prison. After returning to British's via Switzerland and Gibraltor, he used his experience to command an operation that rescued many allied soldiers from behind enemy lines.

Neave served on the prosecution staff at the Nuremberg war trials and served writs on Rudolph Hess and other defendants. Years later, he persuaded the Soviet custodians of Spandau prison in East Germany to release the aged Hess on "humanitarian grounds."

Neave, who had served in Parliament since 1953, straunchly opposed demands that British troops be withdrawn from Ulster.