The Rail Maritime and Transport (RMT) union is warning that Tube workers could take industrial action in a dispute over pay.
It claims more than 6,000 of its members are being "short changed" by two private companies in charge of London Underground's (LU) tracks and signalling.
The union is calling for staff employed by Metronet and Tube Lines to have a pay increase of 0.75%, which was awarded earlier this year, back-dated 16 months to bring them in line with LU workers.
Bob Crow, the general secretary of the RMT, said legal advice obtained by the union made it clear the companies had to honour an agreement brokered by Ken Livingstone.
The London mayor stepped in last year to avert a series of strikes by announcing that the pay dispute would be resolved through mediation.
Mr Crow said LU had given its employees the back-pay so "now it is up to Metronet and Tube Lines to pay what is owed to the workers, handed over to the private sector since the deal was made".
He said: "We will be making a tribunal application for unfair deduction of wages and we reserve the right to ballot our members for industrial action should the companies continue to drag their feet.
"We said from the start that Tube privatisation would be a bad deal for everyone in London. But there is no way we will accept the creation of a two-tier workforce."
Metronet and Tube Lines have agreed to back-date the pay, but only to when they took over under the public private partnership of LU earlier this year.
Paul Emberley, from Metronet, told BBC London: "It's quite straightforward because we did not own the company, Metronet did not acquire the company until the 4 April this year.
"I guess it is a bit like saying you've moved into a new house and expect someone to take on the bills from the previous owner."