Elizabeth line staff are set to strike on New Year's Eve, with Transport for London saying services will run but will be much busier than usual.
Rail, Maritime and Transport (RMT) staff based in the control room will take the industrial action for 24 hours from 9pm.
RMT members would additionally refuse to work rest days or overtime from 5 December until 18 December, the union said.
Staff are seeking an improved pay offer and improvements to holiday entitlement and a reduction in working hours.
There is free travel on the TfL network between 11.45pm on New Year's Eve and 04.30am on New Year's Day - and there is expected to be high numbers of passengers on the network.
Union general secretary Mick Lynch said: "Our members play a vital role in running the Elizabeth line and their demands for fair treatment on working hours and leave remain unmet.
"The current offer falls short, and our members are determined to secure a fair deal."
A TfL spokesperson said: "We encourage the RMT and MTR (the company operating the Elizabeth line) to find a solution to this dispute."
Earlier this month, planned strikes by RMT staff on the London Underground were called off after talks over pay with TfL.
Workers were set to take action on several days in November but a union spokesperson said it had received 'a significantly improved offer'.
Then, TfL said it believed its offer was 'fair, affordable, good for our colleagues and good news for London'.
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