Key parts of the U.S. position in critical textile negotiations were leaked to a foreign government, undercutting efforts to provide protection for the domestic textile industry, the chief negotiator said yesterday.

Congressional sources said the information could have come only from someone still in government or who had left recently. Sources in the administration and on Capitol Hill refused to speculate on the leaker's identity.

Negotiator Charles R. Carlisle said he discovered that the U.S. negotiating position had been compromised when he made stops in Hong Kong, South Korea and Taiwan to describe the U.S. position in negotiations to renew the multifiber arrangement (MFA), which regulates textile trade. The three nations are leading Far East suppliers of textiles and clothing to the U.S. market.

Carlisle said, however, that the U.S. position was already known in at least one country. He declined to identify the country, although Capitol Hill sources said it was the British crown colony of Hong Kong.

"Certain sensitive information became known to some foreign governments before I conveyed it," Carlisle said in an interview. "It's a problem in this city, a very regrettable problem, that undercuts our ability to conduct the nation's business."

Word of the leak angered Sen. Strom Thurmond (R-S.C.), chairman of the Senate Judiciary Committee and a leading advocate of textile trade restrictions.

Thurmond introduced legislation to forbid government employes from working for any foreign government for two years after leaving U.S. service, and to ban them from lobbying the U.S. government on any issue for one year. Certain high-ranking officials would face a lifetime ban against representing foreign interests.

A hearing is scheduled today in the Judiciary Committee on Thurmond's bill.

As a textile-state senator, Thurmond has long fought for greater restrictions on imports. He is a prime sponsor of a bill, vetoed last year by President Reagan, that would place major restrictions on textile imports to protect domestic producers.

Carlisle is negotiating the renewal of the multifiber arrangement, which expires July 31, under the pressure of a congressional effort to override the Reagan veto. Lacking the two-thirds majority needed to override, congressional supporters have put off the attempt until Aug. 6. They hope to gain added votes if the administration fails to win a MFA agreement that is tougher on imports than the present one.

This is not the first time in recent years that important negotiating information has leaked outside the government. The major pro-Israel lobby group, American Israel Public Affairs Committee (AIPAC), received a copy of a classified report from the International Trade Commission recommending negotiating strategy for a U.S.-Israeli free trade agreement. The report was returned after the FBI opened an investigation.

Today's hearing comes as Congress is paying greater attention to former high officials lobbying the U.S. government on behalf of foreign and domestic clients.

The leak of information on textile negotiations, however, shows that the problem reaches beyond the high-profile, high-priced lobbying by former White House aide Michael K. Deaver that presently is getting most of the attention.

"I think we have a continuing problem in this city," said Car- lisle, "and I think it is widely recognized that exceedingly confidential Cabinet discussions get out on the street within a matter of hours."

The Japanese Embassy, for instance, learned important information in Cabinet deliberations of a trade case involving machine tool imports three years ago. Prime Minister Yasuhiro Nakasone made a personal plea to President Reagan to change the decision.

The administration has been trying to combat this problem, government sources said, by limiting attendance at some meetings to the Cabinet member, an undersecretary and the White House staff. Lower-ranking administration officials complain, however, that lobbyists learn of the decisions before they do.

"It really undermines confidence of the government in trade negotiations," said Sen. David L. Boren (D-Okla.), who will testify at today's hearing on behalf of a bill he has offered that forbids former government officials from representing foreign interests for five years. He said he will propose extending the ban to former members of Congress.

"How can we have confidence that our side is bargaining for us when our negotiators can go and start working for the other side?" asked Boren.

"There is something very disquieting to me," said Thurmond, "about high-level officials leaving the service of the federal government and going to work assisting, advising, lobbying, or in any way representing a foreign power for compensation."

He said such a ban could prevent other countries from learning U.S. trade strategy or defense postures "from former officials whose knowledge of those issues could do harm to this country if it is conveyed to others."

Thurmond's bill would place a lifetime ban on the representation of foreign interests by high-level federal officials -- including Cabinet secretaries, the CIA and FBI directors, secretaries of the armed services, the U.S. trade representative and high-ranking White House aides.

Other federal employes -- including former members of Congress, retired military and officials of government-established corporations -- would be restricted from lobbying the federal government for one year and representing foreign interests for two years.