Lech Walesa is under investigation for possible tax evasion and mismanagement of Solidarity union finances but will not face criminal charges, the Polish government's spokesman said yesterday.

Jerzy Urban, at a news conference, repeated official explanations that Walesa had been taken for questioning on alleged financial misdeeds when he disappeared mysteriously last Thursday, hours before he had been expected to speak in the Baltic port of Gdansk.

The Polish Communist Party newspaper Trybuna Ludu said in a commentary Monday that Walesa was questioned on personal and union finances when he was detained last Thursday, but did not give the details supplied by Urban.

Urban said that government investigators "audited the Gdansk chapter of Solidarity and found grave wrongdoings in chapter accounts. As Mr. Walesa was chapter chairman and in charge of finances and signed certain financial documents, he was questioned about these affairs.

"Mr. Walesa has not filed the required documents for a tax audit," Urban added, explaining that he believed the years in question were 1980 and 1981, before Walesa began an 11-month martial law internment.

Walesa described the allegations as nonsense. Contacted by Reuter at his Gdansk home, he said: "They have absolutely no grounds for questioning me about my tax."

In other developments:

* European Community officials decided to extend indefinitely trade sanctions against the Soviet Union imposed last March in condemnation of its role in the military repression of Poland.

* U.N. Secretary General Javier Perez de Cuellar, acting on a Human Rights Commission request, named Hugo Gobbi of Argentina to watch the situation in Poland and report back by March.