The comments, made in a handout by the IG Metall union to staff, come a week after the group's management expanded the board by two seats against despite labour opposition, which unions called a break with tradition.

Workers have traditionally commanded great clout at Thyssenkrupp, and hold half of the 20 seats on the group's supervisory board, but Chairman Siegfried Russwurm last week used his vote, which counts twice in case of a tie, to push the change through.

Unions said this was a first at the company, which under current Miguel Lopez seems to be taking a less diplomatic approach to its relationship with workers.

"Lopez is trying to push through his agenda ruthlessly - even against IG Metall," said Juergen Kerner, deputy head of IG Metall and deputy chairman of Thyssenkrupp's supervisory board.

Thyssenkrupp is currently in discussions to divest both its steel and marine divisions, steps which should not be decided with Russwurm's decisive vote, said Tekin Nasikkol, head of Thyssenkrupp's works council and a member of the group's supervisory board.

"I warn against further attacking the tried and tested social partnership," he said in the leaflet seen by Reuters and dated Dec. 8.

Thyssenkrupp declined to comments and referred to comments by Lopez and Russwurm last week, when the board expansion was disclosed.

(Reporting by Tom Kaeckenhoff; Writing by Christoph Steitz; editing by David Evans)