The RMT wants pay rises for its members

The Rail Maritime & Transport (RMT) union is already balloting workers at Network Rail in a row over pensions with the result due on Monday.

Now union leaders have declared they are in dispute with LU and the private maintenance firms after failure to agree pay deals.

About 9,000 members are voting with the result expected in early June.

The RMT said LU and the private companies had failed to make offers "remotely approaching" the union's aspirations.

Union leader Bob Crow said: "LU has refused point blank to discuss elements of our pay claim and their offer of 3% this year is conditional on unacceptable changes to working practices and our hard-won working conditions.

"We have shown a great deal of patience and we remain prepared to consider a long term deal but the reality is that we are being offered no deal at all and we are not prepared to be strung along simply because there is a mayoral election taking place next month."

The RMT is seeking a pay rise in each year of any long term deal and wants its members to work a 32 hour week over four days.

But an LU spokesman said: "We are surprised the RMT is proposing industrial action at this time as we are still in negotiations with them."

Meanwhile, 2,000 Network Rail station staff and office workers belonging to the Transport Salaried Staffs Association will also be balloted for industrial action because of a row over pensions.