The arch-conservative Christian Social Union (CSU) warned Kohl's CDU they were unalterably opposed to the idea of forming a ``grand coalition'' with the opposition SPD after the federal election in three weeks.

``That would of course mean the end of our relationship if the CDU were to enter a coalition with the SPD,'' said CSU parliamentary leader Michael Glos in an interview with German Radio.

``There is no way that we would be part of any alliance with the SPD,'' said Glos, whose party is fighting hard to defend its absolute majority in its Bavarian conservative bastion in an important state election on Sunday.

Kohl reacted quickly to the comments. Speaking in Berlin, he said it was unthinkable that the CDU would split with the CSU and form a coalition with theSPD. Kohl has said he would resign before joining a grand coalition with the SPD.

``Everyone knows it is inconceivable,'' Kohl said. ``The CDU and CSU have enjoyed a close and proven alliance.''

With three weeks left before the federal election, voter surveys show Kohl's CDU and their Bavarian sister party trailing SPD challenger Gerhard Schroeder by between three and six percentage points.

The SPD is unlikely to win an absolute majority and will need a coalition partner. The Greens would be the first choice, but Schroeder has tried to woo middle-of-the-road voters by flirting with the idea of a grand coalition with the CDU.

Germany was governed by such a grand coalition between 1966 and 1969. It marked the end of the CDU/CSU's 20-year hold on the chancellery and led to SPD-led governments for 13 years.

Although the CSU and conservative members of Kohl's CDU oppose a grand coalition, moderate CDU leaders have said such an alliance may be the only way to resolve many of the country's pressingeconomic and social problems.

``The CDU has to understand we are not stuck together `until death do us part', '' Glos said. ``The CSU is not afraid of going into opposition. I don't believe the CDU will go off on its own. We will govern together or go into the opposition together.''

Schroeder said that he would be willing to form a coalition with the Greens or the CDU after the election. He said it wouldn't matter if the CSU joined the coalition or not.

``It doesn't matter to me if parts of Kohl's conservatives want to sulk off on their own,'' he said at the weekend. ``The only thing that matters is that we have a stable majority.''

A closely watched voter survey by the Electoral Research group showed the CDU trimming the once-towering SPD lead to three points: 38 to 41 per cent. It was the slimmest margin since February. Schroeder had led by eight points in May.

Two other weekend polls, however, showed Schroeder widening his lead. A survey by the Emnid polling institute found Schroeder ahead byfive points, 42 to 37, while another poll by the Dimap institute had Schroeder up by six points, 43 to 37. Kohl came under renewed pressure on Monday from his other coalition partners, the liberal Free Democrats.

FDP chairman Wolfgang Gerhardt said if Kohl, 68, won a fifth term he should not remain in office for the full four years, but ought to make way for a younger leader from his party. ``It's important for the FDP that the chancellor hand over the reigns of power sometime during the next four years,'' Gerhardt said.