A company has been sacked from a multi-million pound contract to clean the Tube after it was found hundreds of workers were being underpaid.
The union is taking legal advice on lodging a claim for back pay
About 400 cleaners were paid £5.05 an hour, instead of the agreed £5.50.
Metronet, responsible for maintaining most Tube lines, said actions of the Blue Diamond firm were "inexcusable".
A spokesman for Blue Diamond said it was "disappointed" to lose the contract and it had always intended to work towards the £5.50 an hour minimum wage.
The contract will be terminated on Thursday, and a new firm will be hired to complete the three-year contract, thought to be worth £30m.
The Transport and General Workers' Union said the firm had robbed workers of thousands of pounds.
Jack Dromey, the union's deputy general secretary, said "Metronet have done the decent thing, acting promptly and properly to terminate the contract.
"This company did the dirty on the cleaners and is paying the price."
Andrew Lezala, chief executive of Metronet, said the contract was signed last October but workers only realised they were being paid the incorrect amount in recent weeks.
He added: "Metronet have adopted a set of values and behaviours which we expect our partners to embrace. We will not tolerate contractors who fail to meet these standards."
Mr Lezala said he did not know if the employers were being greedy or were trying to cover costs it had not anticipated.
The union is taking legal advice about the possibility of lodging a claim for back pay.