Fifty-five MPs of two opposition parliamentary groups, the socialists' Coalition for Bulgaria and the Movement for Rights and Freedoms, petitioned the Constitutional Court about recent amendments to the Labour Code.
The MPs contest provisions which introduce fines for employees working without labour contracts, state-run Bulgarian News Agency (BTA) reported.
They also challenge amendments that introduce higher criteria for trade unions and employers' organisations wishing to participate in the National Council for Tripartite Cooperation which also includes government officials.
Under the new legislation, one of the big employer organisations in the country, the Bulgarian Chamber of Commerce and Industry, would not qualify for participation in the council.
The amendments were vetoed by former President Georgi Purvanov at the end of December but Parliament rejected the veto on January 19.