Two of Mikhail S. Gorbachev's economic advisers have broken with him and are echoing the liberal opposition in calling the Soviet leader's plan for converting to a market economy seriously flawed and inflationary.

Nikolai Petrakov, Stanislav Shata-lin and 11 other economists warn in a letter that Gorbachev's plan, adopted Oct. 19 by the legislature, will raise the budget deficit and disrupt economic links among the 15 Soviet republics.Gorbachev's compromise program for restructuring the Soviet economy is already under sharp attack in many of the republics, and the addition of criticism from his own advisers puts in serious doubt whether it will succeed.

"The decrease in the standard of living of the population will be significant before stabilization. . . . The inflationary spiral is winding up too quickly, the collapse of the consumer market is growing and the economy is moving further from a state of equilibrium," the economists say.

Their letter was published Sunday in the liberal newspaper Komsoml-skaya Pravda. In it, they accuse the Soviet leadership of "indecisiveness and inconsistency" in economic reform, and envision a drop in people's standard of living if the Gorbachev plan is pursued.

The statement constituted a rare public airing of differences between Gorbachev and his aides on how to rebuild the economy to make it efficient, competitive and able to meet basic consumer needs.

Shatalin, a member of Gorbachev's advisory Presidential Council and author of a plan for switching to a market system in 500 days, said immediately after the Gorbachev plan was adopted that he supported it generally.

In their criticism, the economists appeared to side with Boris N. Yelt-sin, president of the Russian republic.

Yeltsin has described Gorbachev's plan as a "catastrophe" that is "doomed to fail" within months after driving up prices and the budget deficit and chopping the ruble's purchasing power.