Polish authorities, in a new effort to discredit Lech Walesa, aired on national television tonight an alleged tape recording of him planning to invest a million dollars in the Vatican Bank.

The recording was said to be a conversation between Walesa and his half-brother, Stanislaw, which Lech was said to have taped on Sept. 29, 1982, at Armalov where the leader of the banned Solidarity labor union was interned.

A text of the alleged conversation was circulated months ago among western journalists, but tonight's was the first broadcast. One of the voices did sound like Walesa, but the quality of the recording was poor and the train of conversation seemed erratic.

Walesa watched the broadcast at his home in Gdansk but refused comment afterwards except to say that he had called the recording a forgery earlier this year.

The careless remarks attributed to him on the tape contrast with his normal circumspection about recording devices.

In the recording, the purported voice of Walesa, in language salted with expletives, is heard urging the brother to deposit $1 million in a 15 percent interest-bearing account in the Vatican Bank. How the money was accumulated or where it was to be transferred from is never specified.

The program began and ended with shots of Walesa appealing to authorities last month "to stop talking about my millions," insisting he has no western bank accounts and denying insinuations that he had appropriated money as Solidarity chief.