WARSAW, OCT. 26 -- The banned Solidarity trade union today called on Poles to boycott a government referendum next month on proposed economic and political reforms, saying "society should not take part in an undertaking of a purely propaganda nature."

The protest call, contained in a statement signed by Solidarity leader Lech Walesa, posed a potentially serious obstacle for the government of Gen. Wojciech Jaruzelski in its effort to mobilize a long-apathetic society through the referendum campaign.

The move also indicated that the longstanding impasse between Jaruzelski's leadership and the Solidarity-based opposition will not be affected soon by the general's decision to pursue a new reform program. "One should remember that the country is ruled today by the same group that six years ago quashed society's democratic aspirations by introducing martial law," Solidarity said.

"The ruling group pledged at that time to introduce economic reforms and pull Poland out of its crisis, but none of these promises was kept and nobody believes the authorities anymore."

The Polish parliament approved two questions last Friday on which Poles are to vote Nov. 29: a "radical curing" of the economy and a communist "model of democratization." Government strategists see the referendum as a way of winning a mandate for painful and potentially explosive measures that are to be included in the economic program, including sharp increases in prices.

Jaruzelski has pledged that the new political model will include liberalizing reforms, such as more authority for local governments and more competition in elections. Officials have hinted in recent months about talks with Solidarity advisers and efforts to recruit opposition activists for new advisory bodies.

At the same time, officials have made clear that the reform measures stop short of recognizing independent political or trade union organizations. Official propaganda has continued its regular attacks on Walesa and other opposition leaders, depicting them as "extremists" and "paid agents" of the Reagan administration.

In a cautious statement drawn up before the announcement of the referendum questions last week, Walesa appeared to leave room for dialogue with the government, saying Solidarity supported reforms and that the referendum could be a positive step, if "honestly thought out and honestly carried out."

Today, however, Walesa charged that the questions were so vague and empty of choice that the referendum could be little more than a propaganda instrument. "To the question of whether we should take part, we are forced unequivocally to say no," the statement said.

The union's call appeared to greatly increase the risks of the vote for the government. Previous calls by Solidarity for boycotts of elections, in 1984 and 1985, resulted in absenteeism reported by the government at 22 to 25 percent, and by the union as far higher. At the same time, according to the new law governing referendums, a referendum proposition is binding only if more than 50 percent of all eligible voters vote "yes." Thus a "no" vote of merely 25 percent combined with absenteeism on the level of the past two elections apparently could defeat the government propositions.

Solidarity's positioning follows weeks of debate within the movement over how to shape its activity and strategy to the emerging climate of reform here. Key activists say the union must move away from the tactics of its stubborn underground battle to survive under tough government repression and take advantage of the more liberal conditions prevalent since an amnesty last year that effectively ended the large-scale imprisonment of opposition members.