The IRA, firmly quashing any speculation about a cease-fire, has shown it is far from a spent force by launching a daring mortar attack on British Prime Minister John Major at the height of a gulf war security alert.

The assassination attempt Thursday was a major propaganda coup for the Irish Republican Army in its fight to oust Britain from Northern Ireland.Cmdr. George Churchill-Coleman, head of Britain's police anti-terrorist squad, conceded after the attack on Major's residence: "You are dealing with a versatile and sometimes cunning organization."

It was the IRA's resounding answer to growing speculation that it was contemplating a cease-fire in one of the world's longest-running guerrilla conflicts.

At least 600 IRA activists are in prison in Northern Ireland, and electoral support for its political wing, Sinn Fein, has foundered. Some of its most experienced guerrillas were arrested last year in Britain and Europe.

But the strike against the heart of the British political establishment, meticulously planned since Margaret Thatcher was in power, highlighted the IRA's ability to bounce back. New volunteers are quick to step into the breach and join the tight-knit group, which keeps the number of its front-line guerrillas to 250 and is now better armed than ever before.

It launched its attack on Major almost seven years after coming within inches of killing Thatcher with a bomb at a Conservative Party conference in the southern resort of Brighton.

"Remember, we only have to be lucky once. You have to be lucky always," the IRA said in 1984.

Mortar rounds falling just 50 feet from a war Cabinet meeting at No. 10 Downing St. will generate the same worldwide publicity for the group - a morale booster after a string of botched attacks killed civilians and drew criticism even from ardent supporters.

Republicans, noting that Britain is preoccupied with its involvement in the gulf war, quoted the adage: "England's difficulty is Ireland's opportunity."

British police chiefs conceded that the mortar attack was daring and carefully planned - even though the group chose to fire a weapon noted for its inaccuracy.

Although IRA mortar shells have killed 12 police and soldiers in Northern Ireland, they also accidentally injured almost 80 civilians. More than half of the projectiles failed to explode when fired.

"Mortar bombs are the IRA's equivalent of the (Iraqi) Scud - inaccurate, unpredictable, but potentially devastating," said the Irish Independent newspaper.