MOSCOW (AP) _ The chances of a formal split in the Communist Party increased significantly Thursday with the resignation of Eduard Shevardnadze, a reform leader Mikhail Gorbachev would like to keep in the fold.

The former foreign minister renounced his party membership as he angrily refused to submit to an internal investigation of his role in founding a democratic opposition movement.

''Under no circumstances will I submit to such an investigation and kangaroo court, because if I agreed to it, I would be providing support ... to the return of the Communist Party leadership to repressive methods of crushing dissent,'' said Shevardnadze, 63, in his resignation letter.

Boris Yeltsin, president of the Russian Federation, called Shevardnadze's decision a ''personal matter'' but said he believed many others would quit, too.

''That process will go pretty fast now, I think,'' said Yeltsin, who quit the party one year ago.

Fyodor Burlatsky, a prominent Soviet legislator and party member, predicted that the departure of an internationally respected figure like Shevardnadze could prompt the defection of about one-quarter of the party membership, which plummeted from 19 million to 16.3 million last year.

There was no immediate comment from Gorbachev, who indicated in a speech published Wednesday that he was still fighting to preserve the Communist Party's power and keep the reformist wing within it.

Shevardnadze and eight other leading reformers this week founded a democratic reform movement that could evolve into an opposition party challenging the Communists that have ruled here since 1917.

The party Central Control Commission launched an ''investigation'' of his call for an opposition party and demanded he appear before the panel last Monday. He did not show up.

Commission spokesman Konstantin Kachalin said Thursday that the panel discussed Shevardnadze's case Monday but ''since he is quitting the Communist Party, we are not going to take further action.''

The commission could have censured Shevardnadze and recommended his expulsion.

The nine founders of the new democracy movement are all Communists, or ex- Communists turned reformers. Many have been or are still close to Gorbachev, who is both president and Communist Party leader.

Gorbachev has not opposed the new coalition, but in a speech published Wednesday by the Communist Party daily Pravda, he said the conflict between reformers and hard-liners was destroying the party.

''I must say that if this continues, if the party remains in such a condition, it will lose all political struggles and the elections we face in the near future,'' he said, speaking to a committee drafting a new, less Marxist party platform.

''People are simply turned off by the party: what can it give to society, if it cannot even get its own house in order?'' Gorbachev said. ''So the situation, I will say bluntly, is critical, and we must act to straighten it out.''

The party, blamed by many ordinary Soviets for mismanagement and corruption, suffered a stinging defeat in the Russian republic's June 12 presidential elections. Yeltsin trounced three Communists.

In one of Gorbachev's main reforms, the party relinquished its constitutional monopoly on power in March 1990. But it retains a firm grip on the country's military-industrial complex and much of the rest of the economy, and no viable opposition party has emerged.

Shevardnadze, a member of the party's policy-making Central Committee, came under attack in the hard-line Communist media last month after he said the country would benefit from strong competition to the party.

Those remarks helped launch the democratic movement and also set off the party investigation.

The Communist Party has a history of attacking its critics, ranging from executions under Josef Stalin to more recent accusations of fraud against successful businessmen and allies of Yeltsin.

Shevardnadze, one of the architects of Gorbachev's perestroika reforms, was one of the president's closest allies and a full member of the party's ruling Politburo until July 1990. Before becoming foreign minister in 1985, he headed the Communist Party in his native Georgia.

After helping to end the Cold War, he abruptly announced his resignation as foreign minister last December, warning that the country was heading toward dictatorship as hard-liners gained influence.

Burlatsky, who directed Gorbachev's release of political prisoners and served as Nikita Khrushchev's speech writer, said Thursday that he, too, was thinking of quitting the party.

Burlatsky said Alexander Yakovlev, a close Gorbachev ally who is another founder of the democracy coalition, also might leave the party. Yakovlev is widely considered the architect of Gorbachev's policy of glasnost.