LONDON--Britain broke relations with Syria yesterday, charging that Syrian officials conspired with a Jordanian who was convicted hours earlier of trying to blow up an Israeli jumbo jet by smuggling a bomb on board in London.

Syria called the action unjustified and ordered its air space and territorial waters closed to British planes and ships.

Foreign Secretary Sir Geoffrey Howe told Parliament there was conclusive evidence of official Syrian involvement in the "monstrous and inhumane" attempt to sabotage the El Al flight to Tel Aviv and kill the 375 people aboard.

A jury at the Old Bailey criminal court convicted Nezar Hindawi, 32, earlier yesterday of trying to smuggle a bomb onto the plane at Heathrow airport in the hand luggage of his pregnant Irish girlfriend.

Judge William Mars-Jones sentenced Hindawi to 45 years in prison. He said terrorists "can expect no mercy from our courts... A more callous and cruel deception and a more horrendous massacre is difficult to imagine."

Within an hour of the verdict, the Foreign Office summoned Syrian Ambassador Loutof Allah Haydar and told him of Britain's intention to expel him and the 20 other diplomats in the Syrian Embassy.

Howe said in his announcement to the House of Commons that the evidence against Syria included testimony that Hindawi carried an official Syrian passport and met with the Syrian ambassador on the day the bomb was found.

"In addition, we have independent evidence that the Syrian ambassador was personally involved, several months before the commission of the offense, in securing for Hindawi the sponsorship of the Syrian intelligence authorities," he said in the speech less than three hours after Hindawi was found guilty.

Syria has been ordered to close its embassy within 14 days, Howe said, and the British Embassy in Damascus also will be closed.

More rigorous checks will be made on passengers, crew and baggage of Syrian Arab Airlines' flights arriving in Britain, the foreign secretary said. Britain stopped short of banning the airline.

Howe said Britain will "maintain and strengthen" visa restrictions on visiting Syrians.

It is Britain's second break with an Arab country in two and a half years. Prime Minister Margaret Thatcher's government ended relations with Libya in April 1984 after gunfire from the Libyan Embassy killed a London policewoman during a demonstration by Libyan exiles.

A Syrian official in Damascus called Britain's action a "hostile campaign" against his country, and state television said the British Embassy would be closed.

The Syrians said the British Council complex, a cultural center supported by the British government, also would be shut down and Syrian air space and territorial waters would be offlimits for British aircraft and ships.

Ambassador Roger Tomkys, 18 other diplomats at the embassy and British Council personnel were given one week to leave, the report said.