British and American defense officials have had exploratory discussions on whether the two countries should develop a modern chemicalwarfare capability to counter what they believe to be a substantial Soviet stockpile of nerve gas and other poison chemical weapons.

The talks began in Washington last year at the initiative of then British defense secretary Francis Pym, who has been outspoken in his concern about the threat posed by the Soviet Union's "rather sophisticated chemical capability" and the deadlock in U.S.-Soviet negotiations for a complete ban on chemical warfare.

British officials said the Anglo-American talks have examined the Soviet chemical warfare threat and what the United States, Britain and possibly the NATO allies could do in response. Both British and American officials stressed that no decisions have yet been made. b

Officials here said Britain still has no plans to resume producing chemical weapons, a capability it gave up after experiencing the horrors of poison gas in World War I. Instead, they indicated, Britain is waiting to see what the Reagan administration decides to do in the United States, which also stopped producing chemical weapons and began destroying much of its stockpile of them in 1969.

"Both sides of the Atlantic seem to be waiting on the other," said one well-informed source here. "London is waiting for the Reagan administration to make a decision, while it looks like Reagan's people will be waiting to see what the Europeans think."

This source, a British expert on chemical warfare, added that U.S. research into the development of chemical weapons -- particularly binary nerve gas, in which two chemicals, safe while stored separately, become lethal when mixed -- "seems quite far advanced and ready to go." He said Britain and the United States have shared their chemical warfare research, much of it done to develop defense measures like masks and protective suits, during the years when neither country has produced new chemical weapons.

Congress has asked the incoming administration to report early this year on the possibility of U.S. production of binary or other chemical weapons at a plant that would be built in Pine Bluff, Ark. The Senate in September approved a $3 million appropriation for a building to house such a plant there.

But British officials strongly denied growing speculation here that the talks between British and American defense officials and Pym's several recent public statements here on chemical warfare meant the two countries were already close to agreement on producing and deploying binary nerve gas or other chemical weapons.

"This has almost gotten out of hand," Pym said today in an interview. "These were informal talks at staff planning level between us and the Americans. Nothing was decided. Nothing has happened at all.

"As far as I know, neither the present U.S. administration nor the new administration has come to any conclusions about this," added Pym, who last week was moved from defense minister to leader of the House of Commons by Prime Minister Margaret Thatcher.

Because of the deadly experience of British troops with poison gas in the trenches of World War I, chemical warfare is an emotional subject in Britain. This has made Pym's carefully couched remarks highly controversial and heightened concern here that Britain might be joining in or agreeing to American development of chemical weapons.

Pym said his aim has been to make the British people and other allies aware of the threat he sees in the Soviet chemical weapon stockpile. "This threat does exist," he said. "Some people want to brush it under the carpet, but I think it's important that we realize that it exists. I wanted to discuss with the Americans possibilities for a deterrent of some kind."

British defense officials and diplomats still support U.S. efforts, also begun at British initiative in the East-West disarmament talks in Geneva in 1976, to negotiate a chemical warfare ban with the Soviet Union. But they see little hope now of breaking the deadlock over the problem of verification.

"There's been no movement on the other side," said Pym in one of his recent public discussions of the issue, "and the fact is that this rather sophisticated chemical capability does exist, is in Soviet hands and therefore is a threat to us."

British and American officials said that much of the emphasis Pym has put on the problem appears to stem from his "strong personal feelings," but they added that they expected further Anglo-American discussions and possibly informal consideration of the issue by NATO defense ministers this spring.

Experts here divide over how extensive and up-to-date the Soviet chemical weapons are and whether they have been used in the invasion of Afghanistan, as has been alleged by some Afghans. There is little question that the Soviets at least matched U.S. production through the late 1960s of nerve gas, choking agents, blood poisons and other lethal chemicals for military use.