British Prime Minister Margaret Thatcher urged fellow European leaders today to strengthen international safeguards against terrorism following the hijacking of a TWA airliner to Beirut.

The informal British proposals, including tougher sanctions against countries disregarding international obligations to combat air piracy, were discussed by leaders of the 10 European Community countries at a regular summit meeting here. Senior European officials also were considering a draft declaration on possible measures against terrorism.

Diplomats attending the summit said there was doubt whether all member countries would endorse the British approach. The French government, in particular, seemed skeptical about the value of multilateral declarations on terrorism while sensitive, behind-the-scenes negotiations are still in progress with Shiite Moslem militiamen in Beirut.

Besides the 39 American hostages from the TWA plane, four French citizens and seven other Americans are being held hostage in Lebanon.

The new Middle East tensions gave European leaders a live political crisis to discuss at a summit meeting originally seen as a major step toward relaunching the ideal of a united Europe. The European Community has been paralyzed for much of the past five years with member states clinging to the defense of national interests and unable to agree on long overdue structural reforms.

France and West Germany proposed a project for European union today to boost coordination between EC states on foreign policy and security issues. The idea has been criticized as "insufficient" by more federalist-minded countries like Italy and the Netherlands, which would like speedier progress toward full political integration.

Outlining the British ideas on combatting terrorism, a spokesman for Thatcher called for measures to ensure that hijackers paid for their actions, and for the strengthening and extension to more countries of existing international conventions against terrorism and air piracy.

The hijacking crisis in Lebanon is a particularly delicate one in view of the presence here of Greek Prime Minister Andreas Papandreou, whose government has been attacked by Washington for lax security measures at Athens Airport. Officials said that there was a tacit understanding here not to criticize Greece publicly as long as the present hostage crisis continues.

The discussion of terrorism at the Milan summit has coincided with a European tour by Vice President Bush that has become dominated by the hostage crisis. Bush, now in Geneva, is due to fly to France Saturday and to Britain on Monday.

Differences of opinion on how to deal with terrorism were revealed by a senior French official, who said he doubted whether the European summit was the place to tackle the problem. He said that bilateral negotiations and a tough stand by individual governments was more likely to produce results.

The French official described Bush's emphasis on consulting European allies about terrorism as largely dictated by domestic political considerations. He said France already cooperated closely with the United States in this field.

Multilateral action against terrorism has been considered at successive meetings of western political leaders. In 1978, the Bonn economic summit called for sanctions against countries harboring or refusing to punish hijackers, but the only country ever singled out for retaliation was Afghanistan in 1981.