A bomb set to explode in midair was found by Israeli security guards at London's Heathrow airport this morning in the baggage of a woman about to board an El Al flight to Tel Aviv.

Police said the woman, an Irish national who was taken into custody, may have been duped into carrying the bag by her boyfriend, a man "of Arab origin." They said the man, who arrived with her at the airport but disappeared before the bomb was discovered, was "known to her" by the name of Nezar Hindawi, but they did not provide his nationality.

While there was no public evidence to link the attempted bombing to Tuesday's attack by U.S. planes on Libya, many here believed it to be an attempted retaliation for the use of British air bases to launch the U.S. strike. Its discovery this morning coincided with news that three British hostages held by extremists in Lebanon had been killed, another British citizen had been abducted in Beirut and a rocket attack had been launched against the residence of Britain's ambassador in the Lebanese capital.

The terrorist acts heightened a sense of anxiety among many Britons over the implications of the air attack on Libya. There was widespread mention of the fact that Britain, rather than the United States, had been the first to feel the brunt of retaliation.

In Parliament, news of the airport bomb and events in Lebanon brought further criticism of Prime Minister Margaret Thatcher. In an angry exchange this afternoon, opposition Labor Party leader Neil Kinnock accused her of "provoking terrorism" by giving British approval and assistance to the U.S. attack.

Liberal Party leader David Steel said that by backing the U.S. plan, Thatcher had exposed British citizens to retaliation by Libyan leader Muammar Qaddafi and those he supports.

"That's what happens when you turn the British bulldog into Reagan's poodle," Steel said.

As opposition politicians shouted that Thatcher now has British "blood on your hands," the prime minister's supporters on the Conservative Party benches rose to shout in her defense.

"If you let the threat of further terrorism prevent you from fighting against it," Thatcher said, "then the terrorist has won and he will hold you to ransom."

Several relatives of the Britons killed in Lebanon referred in interviews to ongoing negotiations for their release, and said that they held Thatcher's decision responsible for the deaths.

Government spokesmen said that Britain had "taken account of the position of our hostages in the Middle East" in deciding to back the United States. A fourth British citizen, who also has Irish citizenship, Brian Keenan, has been missing since he disappeared in Beirut last week.

In a statement tonight in Paris, where he had attended an emergency meeting of European Community foreign ministers on Libya, Foreign Secretary Geoffrey Howe said: "I totally and fully understand the anguish of those families whose members are caught up in the horrors of terrorism. But if we are to prevent others from having that experience, we must not concede victory to the terrorist."

As the country girded itself for further retaliatory attacks, British bases in the Mediterranean have been reinforced, and security has been stepped up in the past 48 hours. Additional equipment, including C130 Hercules transports for possible evacuation of British nationals in Lebanon and Libya, reportedly have been flown to British bases in Cyprus, and at least two warships were believed to be standing by in the area.

In a statement read tonight over the BBC World Service broadcasting to the Middle East, the Foreign Office advised the approximately 70 British citizens in west Beirut to stay in their homes and keep in touch with the British Embassy there. The broadcast listed two embassy telephone numbers and advised Britons to continue to listen to the service for further announcements.

Already tight security at Heathrow airport was stepped up today following discovery of the bomb there, which apparently slipped through routine X-ray checks by airport personnel before being discovered by Israeli guards.

The discovery took place just before 9:15 this morning, local time, when a woman passenger arrived at the boarding gate for El Al Flight 016, en route from New York to Tel Aviv with a stop in London. She told police later she was flying to Israel for a vacation.

In January, following terrorist attacks in El Al check-in areas at the Rome and Vienna airports, Heathrow authorities moved boarding gates for El Al from Terminal 3, where most international flights are handled, to Terminal 1. Terminal 1 is more easily isolated from the rest of the airport, and handles flights to and from Northern Ireland's capital, Belfast, believed to pose a similar high security risk.

Like passengers to Belfast aboard British airlines, El Al passengers do not check in at the terminal departure area, but must carry both hand luggage and baggage to be checked to the departure gate. En route, they stop at an immigration desk and pass into the international departures area through a standard security check, including a personal metal detector and a baggage X-ray.

At the El Al boarding gate, they are subjected to another body search and examination of their baggage by El Al security. It was during this check that the bomb was discovered. Described by police sources as weighing less than 10 pounds, it was concealed beneath a false bottom in what British police called "hand luggage."

The woman was seized by British police, who immediately sealed off the terminal and started an intensive but unsuccessful search for an Arab man who had arrived at the airport with her that morning.

Police Commander George Churchill-Coleman said tonight that it was not likely that the woman would be charged. During interrogation, he said, it was determined that she may have carried the bomb "in all innocence."

Describing the bomb as an "improvised explosive device," Churchill-Coleman said it "was viable and would have exploded once the aircraft was airborne. It is highly likely that such an explosion would have resulted in the loss of the aircraft."

Churchill-Coleman said that "no conclusions" should be drawn from the woman's Irish citizenship. He declined to name her, but said that she works and lives in London. Describing Hindawi as her "boyfriend," Churchill-Coleman said she had known him for about 12 months.

The woman, who was said to be pregnant, presumably decided to cooperate with police after learning that she, as well as the approximately 400 other people on the plane, was to be a victim of the bomb.