FRANKFURT, May 23 (Reuters) - Thyssenkrupp on Thursday said its supervisory board approved a planned sale of 20% in the conglomerate's steel division to Czech billionaire Daniel Kretinsky.

In a statement, the German industrial group said board Chairman Siegfried Russwurm's vote counted twice, an instrument that German corporate governance laws allow to break a stalemate.

Labour leaders, which hold half of the non-executive board's seats, warned last week they might not vote in favour of the deal unless there are written assurances regarding jobs and sites. (Editing by Jonathan Oatis Editing by Chris Reese)