Union bosses have called on the government to stop private companies cutting the pay and conditions of public sector workers.

As Labour's spring conference in Cardiff continues, Unison called on the government to ensure a "two-tier" workforce is not created by the increasing involvement of private firms in public services.

Unison, which represents more than 1.3 million public sector workers, released a dossier on Saturday claiming private firms are giving new workers less pay, pensions and holidays than colleagues transferred with privatised services.

Many of the cases involve local government, which is one of the conference's themes, and the union has decided to use the occasion to lobby the cabinet member in charge.

It listed a raft of local and county councils that have transferred services such as refuse collection and cleaning where employers have used new employees contracts as a way of cutting costs.

The union, which is opposed to the government's pledge to make greater use of private firms called on Stephen Byers to honour a pledge at Labour's October conference to outlaw such practices.

"Where evidence of a two-tier workforce is found to exist we will take action to end it," the minister had told delegates.

General secretary of Unison, Dave Prentis, demanded action from the secretary of state for transport local government and the regions.

"Stephen Byers promised action and we expect him to deliver on that promise. He wanted the evidence and we have provided it. And it is pretty damning evidence of how private sector companies are coming into local government and making their profits by cutting the pay and conditions of the workforce," he said.

"It cannot be right that two people employed by the same company doing the same job should be paid on different rates. We are talking about penny-pinching measures which in the main affect adversely women employees who make up almost three-quarters of the workforce. Those most at risk in the emerging two-tier workforce are the already vulnerable - including black and ethnic minority staff who continue to be concentrated in the lower paid jobs."