LONDON, DEC. 21 -- Britain and France, Western Europe's only nuclear powers, are moving toward a new level of defense cooperation that could include joint development of an air-launched, nuclear-armed missile.

Collaboration on the tactical air-to-surface missile would mark the first time Britain has worked with a country other than the United States on nuclear armaments.

France is believed to have taken the lead in promoting a closer relationship as part of its overall effort to strengthen its defense ties with Europe. The move toward Britain follows French efforts to revive the seven-member Western European Union, and to establish closer bilateral defense links with West Germany.

British officials reportedly were quick to insist that their aim in responding to the French overtures to establish a stronger "European pillar" of the North Atlantic Treaty Organization was not to diminish the U.S. role in the alliance or the special Anglo-U.S. relationship.

Instead, one official said, "our underlying position regarding all matters French is that we are very happy to develop our bilateral relationship with them in ways that will strengthen" NATO overall.

Although France remains a member of NATO, it withdrew its forces from the alliance's integrated military command in 1966. Officials here said they understood that the French have no interest in reversing that decision.

But British government sources said London and Paris share a concern over preserving a comprehensive nuclear deterrent to defend Europe, especially in light of the U.S.-Soviet treaty to eliminate intermediate-range nuclear forces (INF) from the continent.

Both Britain and France have rejected Soviet efforts to include their independent strategic, or long-range, nuclear forces in further negotiations between Washington and Moscow on reducing the superpower strategic arsenals. "We have a close common interest in the way French and British systems are treated, or preferably not treated," in these superpower talks, a British official said.

The new ties break a long-standing mutual reluctance to collaborate on nuclear matters. In an unprecedented series of contacts, British Defense Secretary George Younger, and his French counterpart, Andre Giraud, have held at least seven meetings this year.

Their most recent discussions have centered on joint development of the missile, to modernize capabilities in weapon ranges below that of their strategic forces. In a joint press conference on Dec. 14, Younger said Britain was considering the proposal but had made no decision.

France has an operating version of "stand-off" weapons, Air-Sol Moyenne Portee, with a range of 60 to 200 miles, which entered service last year on Mirage bombers.

The French want to develop a new model of the missile that would extend its range up to 300 miles. Ideally, they would like at least one other country to share the expense.

Britain already is modernizing its strategic system, replacing its submarine-launched Polaris missiles with the U.S. Trident system. But it also wants to update, by the late 1990s, its aging arsenal of "free-fall" nuclear bombs carried on Tornado aircraft.

The British maintain that the advanced age of the bombs, as well as improved Soviet fighter aircraft and air defenses, including SA12s and SA10s among some 10,000 surface-to-air missiles ringing the Soviet Union, mandate their replacement with a stand-off system of weapons better able to reach their targets.

The issue of installing a new missile system, particularly one that, in essence, is an air-launched version of the ground-launched cruise missiles to be removed under the INF treaty, could pose political problems for Prime Minister Margaret Thatcher.

Already, opposition Labor Party politicians have accused her of trying to circumvent the spirit, if not the letter, of the INF treaty. But the government argues that it is merely seeking to "restore the status quo" of the air capability it has had since the 1950s.

"It is getting increasingly difficult for manned aircraft to penetrate deep into Soviet territory," one official said. "We would extend the range {of the aircraft} by the range of the missile."

Officials here said they expected no problems with Washington if they decide to go ahead with the Anglo-French project. "You've got to draw a clear distinction between warhead development and weapons systems," said one. "On the warhead side, since 1958, we've been very close to the U.S. -- testing in Nevada, etc. Even if we wanted to, we would find it almost impossible to collaborate with a third party on warheads. But there is no particular mystique about delivery systems," he said.

The United States has been working on its own version of an air-launched cruise missile. But in light of its budgetary problems, U.S. officials have said, Washington is investigating the possibility of a three-way project with Paris and London.

"Everybody has heard the message of the ministries of finance and Congress pretty clearly," said another government source.