The government's labour relations suffered another blow yesterday after union leaders announced a ballot of 180,000 postal workers over a plan to privatise the jobs of 7000 Post Office cleaning and maintenance workers.

The Communication Workers' Union is to declare the result on September 12 - the day that the Fire Brigades Union meet to call their ballot over a claim for a wage of £30,000 a year.

The prime minister is due to address the Trades Union Congress on September 10.