British police agencies around the country today launched a massive search of thousands of resort hotels where Scotland Yard has said the Provisional Irish Republican Army planned to place delayed-detonation bombs.

Describing the search as "one of the largest preventive operations ever mounted," Home Secretary Leon Brittan told the House of Commons today that "a major terrorist outrage [had] been thwarted."

Brittan and others said there was only a slight possibility that bombs, which they said had been targeted for 12 different cities along Britain's coast, actually had been placed as planned. But local police and bomb-sniffing dogs today began to search every hotel room in those cities.

The resort areas, most of which cater to British rather than foreign tourists, are just about to enter their heavy summer season. The IRA plan, police said, was that the bombs would be timed to go off in mid-July.

According to police officials, information leading to the search was obtained from members of "a Provisional IRA active service unit" who have been arrested during the past three days. Fifteen persons have been arrested since Saturday, and are currently being questioned.

The arrests came after a lengthy investigation of the bombing last October of the Grand Hotel in Brighton during the Conservative Party convention. Five persons were killed in the blast, which Prime Minister Margaret Thatcher and much of her Cabinet narrowly escaped. The IRA, which has waged a long campaign of terror to oust British forces from Northern Ireland, claimed responsibility for that bomb.

According to reports here tonight, police now believe the Brighton bomb was placed in the hotel about four weeks before it detonated. Checking the hotel registry, they found a man had registered with a false name about one month before the bombing. The BBC reported tonight that one of those detained over the weekend was the man traced from Brighton.