LONDON, DEC. 20 -- Stunned European leaders, reacting with disbelief and dismay to the resignation of Soviet Foreign Minister Eduard Shevardnadze, said today they fear it could mark a milestone in the rapid deterioration of the Soviet Union into economic and political chaos.

Analysts said Shevardnadze's sudden departure casts a deep shadow of uncertainty on numerous crucial issues, including arms control, the Persian Gulf crisis and Western economic aid to Moscow. Some expressed fear that the resignation could help speed the downfall of President Mikhail Gorbachev.

"I regret this very much," German Chancellor Helmut Kohl, looking grim, told reporters during a break in the pan-German parliament's historic opening session in Berlin.

Asked if the resignation means the end of Gorbachev's leadership, Kohl said, "I don't know if we can speculate at this moment about such forecasts. We hope, we must hope, and I do, that he will pull through."

At North Atlantic Treaty Organization headquarters in Brussels, a senior diplomat told Reuter: "We're stunned. Nobody expected this. This is about much more than one man resigning. It's a major blow to the whole reform process in the Soviet Union."

Simon Lunn, deputy secretary general of NATO's North Atlantic Assembly, said the resignation "has all sorts of nasty implications for future relations . . . . The situation is obviously worse than anyone here thought."

"I am shocked because it's such a sudden and dramatic decision, also because of the reasons behind it, which underscore the graveness and danger of the political conflict going on in Moscow," said Italian Foreign Minister Gianni De Michelis.

French Foreign Minister Roland Dumas said Shevardnadze's resignation speech reflected views that the Soviet minister had often expressed in private. "In tete-a-tete meetings, he never concealed his fears, if perestroika did not succeed, of seeing the rise of dictatorship. He went as far as to speak of a military dictatorship," Dumas told a French radio station. "This should serve as a warning to Western countries that are dragging their feet in the policy of aid to the Soviets."

In Europe, as in the United States, Shevardnadze was seen as the moving force and personification of the policy of openness in foreign affairs. German Foreign Minister Hans-Dietrich Genscher, who met with Shevardnadze 16 times this year, said his resignation "should make all of us in the West recognize that the forces for reform, especially in the economic area, need our deepest support."

NATO Secretary General Manfred Woerner recalled that Shevardnadze's landmark visit to NATO's headquarters last year -- the first by a senior Soviet official -- "really paved the way to a relationship of contacts and cooperation between the Atlantic alliance and the Soviet Union."

Former British prime minister Margaret Thatcher broke a month-long public silence since her own resignation to pay tribute to Shevardnadze, calling him "a statesman of world renown and . . . a great and enthusiastic supporter of the reforms."

Despite a statement from the Soviet Foreign Ministry that the country's Persian Gulf policy will remain unchanged, many in Europe expressed fear that Shevardnadze's departure could damage Soviet resolve and international solidarity. In his parting speech, Shevardnadze called Wednesday's fierce criticism of his gulf policy by Soviet legislators "the last straw" in his decision to resign.

British Foreign Secretary Douglas Hurd sought to minimize the impact of the resignation, saying that while he was personally "very sad" to see Shevardnadze go, "obviously we hope that the policies of reform which he has supported and which President Gorbachev is supporting will be continued. That is important for us and, we think, for the Soviet Union. So we shall be watching this situation with great care and sympathy."

But former French foreign minister Jean Francois-Poncet said the move "confirms that what, as everyone knows, is a disastrous situation is now turning into an open crisis. We have to conclude that the Soviet Union is now heading into the unknown, an unknown that could take dramatic directions."