Southern railway is facing legal action over its decision to withhold holiday pay from workers involved in the conductors' strike.
The Rail, Maritime and Transport (RMT) union said lawyers had advised there was a case to challenge Southern.
Govia Thameslink (GTR), Southern's parent company, said it would withhold pay unless conductors have worked or will work normally during the dispute.
The RMT has been embroiled in a bitter dispute over the role of conductors.
The union told Southern it had received legal advice that withholding backdated holiday pay was in breach of legislation.
'Setting a deadline'
In a message to RMT members, Mick Cash, the union's general secretary, said: "We will inform the company that we now expect these payments to be made to you and your colleagues with immediate effect.
"I will be setting a deadline for the company to respond and if the company continue to deny our members the agreed payments then claims will be placed into early conciliation and employment tribunal proceedings will be lodged with immediate effect."
Further strikes are planned by the RMT in the coming weeks, in the run-up to Christmas and over the new year.
Southern wants drivers, rather than conductors, to operate carriage doors at certain times.
A Southern spokesman said: "Passengers have had to endure 17 days of strikes so far, with 11 more days to come.
"These strikes have caused misery and hardship to people's work and family lives. We feel we cannot make these payments to conductors who are currently taking industrial action."
"The fact they have taken strike action makes the calculation of what is owed more complicated," he added.
"We plan to pay them once the industrial action is over."
RMT strike dates:
- 00:01 Tuesday 22 November to 23:59 Wednesday 23 November
- 00:01 Tuesday 6 December to 23:59 Thursday 8 December
- 00:01 Thursday 22 December to 23:59 Saturday 24 December
- 00:01 Saturday 31 December to 23:59 Monday 2 January