Prosecutors in Chancellor Helmut Kohl's home state said today they are opening an investigation into charges that the West German leader gave false testimony last summer before a parliamentary panel probing illegal party donations.

The decision by Rhineland-Palatinate authorities to open legal proceedings against Kohl followed claims by Otto Schily, a Berlin lawyer and a leading member of the radical Greens Party, that documentary evidence exists showing that Kohl did not tell the truth when he professed ignorance about how some businesses avoided taxes on political contributions by laundering funds through charity front organizations.

Kohl's spokesman, Friedhelm Ost, insisted today that "the allegations raised by Schily are unfounded." Other chancellery officials confirmed that Kohl had retained a Bonn lawyer, Hans Dahs, to handle his defense.

The state investigation, the first in 20 years against an incumbent chancellor, comes 11 months before elections. Kohl's center-right coalition is favored to retain power in next January's vote, but the case could provide an unexpected political windfall for the opposition Social Democrats, who contend that Kohl's tenure in power has been tarnished by scandal.

The Rhineland-Palatinate inquiry is separate from national probes into illegal party financing and the so-called "Flick scandal," in which senior politicians have been accused of taking bribes from the giant Flick industrial conglomerate in return for favorable tax legislation. Former economics minister Otto Lambsdorff, who resigned in 1984, is being tried in Bonn on corruption charges over the Flick affair.

During a 2 1/2-hour interrogation in Mainz last July, Kohl repeatedly insisted that he knew nothing about a tax evasion scheme that allegedly funneled more than $70 million to his Christian Democratic Party between 1969 and 1980. Kohl served as state party chairman from 1966 to 1973 and as state premier from 1969 to 1976.

Kohl told the Mainz committee that he could not recall purported contacts with the managers of several firms who said in written notes that they had consulted Kohl on methods of payment during the period when he served as party chairman and premier in Rhineland-Palatinate.

State prosecutors said that the president of the Bundestag, the lower house of West Germany's parliament, would be informed later this week that a formal investigation was being opened against Kohl. This step is regarded as a necessary prelude to any eventual vote in parliament to lift the immunity enjoyed by a member, if warranted by a criminal indictment.

Kohl's spokesman Ost, said Kohl views the imminent legal proceedings "with equanimity" because he had testified at the Mainz session "to the best of his knowledge and in good faith."

Schily emphasized in a press conference two weeks ago that he had accumulated documents, notes and cash receipts that showed Kohl was clearly aware of the multimillion-dollar scheme to "launder" donations to his party through research foundations with charity status.

Schily, however, has refused to make public any of the material that he claims would prove Kohl committed perjury before a parliamentary inquiry. The crime carries a maximum jail term of five years.

It is still unclear how badly the investigation could affect the fortunes of Kohl and his party in the coming election campaign. Under West German law, the case may be dropped if the prosecutors decide that evidence gleaned in their inquiry is not sufficient to press formal charges.

But the West German newsweekly Der Spiegel reported that there were strong sentiments among Christian Democrats that the party would not enter an important election campaign under Kohl's leadership if he faced a perjury indictment.