The German employees of US giant Wal-Mart Stores are set to go on strike today [Friday] and tomorrow in a bid to pressure the discount retailer into agreeing to a union contract.

The strike, which was called by Germany 's services union Ver.di, is expected to affect at least 40 stores, half of Wal-Mart's German chain.

"Wal-Mart has to find out that Ver.di won't give it any peace until the largest retail company in the world finally acknowledges sector-wide collective agreements at its German stores," said the union. Ver.di secretary Ruediger Wolff told Dow Jones that Wal-Mart should follow the example of other German retailers, including Metro and KarstadtQuelle, who negotiate with the union as part of the employers' organisation.

Meanwhile, the union lodged a complaint with a court in Wuppertal in April, saying that Wal-Mart has failed to post annual earnings figures for its German arm, Wal-Mart Deutschland GmbH. This led to Wal-Mart receiving fines of a total of €5,500 for refusing to release figures for 1999 and 2000.

Wal-Mart released a statement on Wednesday saying that it "has significant doubts about the validity of the law".