WARSAW, Poland (AP) _ Former Solidarity chairman Lech Walesa is under investigation for slander because he disputed voter turnout figures released by the government for last week's parliamentary elections, the government spokesman said Tuesday.

Walesa, winner of the Nobel Peace Prize in 1983 for his leadership of the outlawed Solidarity free trade union, faces up to two years in prison if convicted, said spokesman Jerzy Urban. He said Walesa also could be fined or have his ''freedom'' limited.

''Proceedings have been started against Mr. Walesa, as the source of false information,'' Urban told reporters at his weekly news conference.

He added that Western correspondents who reported Walesa's remarks could be interrogated in the investigation.

Walesa said last week that according to his estimates 45 percent to 47 percent of the voters in his home city of Gdansk cast ballots during the Oct. 13 election. The Solidarity underground later said the turnout in the northern port was 52 percent.

The government, which did not release a separate turnout figure for the city, said the turnout in Gdansk province was 65 percent, the lowest in the country.

Solidarity had called for a voter boycott.

Walesa, contacted by telephone at his Gdansk apartment, said he had not received any summons from the state prosecutor's office in connection with the investigation.

Walesa said Urban's remarks were intended ''to serve one purpose - the quickest way of turning attention away from yesterday's meeting between (Solidarity official) Jerzy Milewski and U.S. President Ronald Reagan.''

On Monday, Reagan criticized increased repressions in Poland following a 15-minute meeting in Washington with Milewski, an associate of Walesa who heads Solidarity's office in Brussels. Reagan said the parliamentary elections ''have failed to provide a genuine public mandate for Poland's legislative representatives.''

Urban had no official comment about the meeting between Reagan and Milewski.

The government said the voter turnout nationwide was 78.86 percent. The Solidarity underground said its separate voter count indicated the turnout was closer to 66 percent or 67 percent.

Since no other organization monitored the results, an independent turnout figure was impossible to obtain.

Urban said the investigation of Walesa followed complaints by electoral officials in Gdansk, Szczecin, Wroclaw, Olsztyn, Elblag, Koszalin and Slupsk.

Walesa also is under a separate investigation for allegedly engaging in illegal union activities. He has been questioned twice by a Gdansk prosecutor in that investigation, which followed a clandestine meeting last February in which Walesa and other Solidarity activists allegedly discussed plans for a strike against food-price hikes.

Urban said Walesa also had been questioned by the management of the Lenin shipyard in Gdansk, where Walesa works, about his travels outside Gdansk while on a 14-day sick leave.

During the time of his leave, Walesa, 42, traveled to Warsaw Friday to attend a memorial mass for the Rev. Jerzy Popieluszko, the slain pro- Solidarity priest, and to Zakopane Saturday to attend the funeral of a Polish general who was a Solidarity supporter.

''This seems to be contradictory to the doctor's diagnosis on the sick leave, which states he is not able to work,'' Urban said.