The RMT wants changes made to track access to be reversed
Hundreds more London Underground workers are planning to strike.
The Rail, Maritime and Transport (RMT) union said 500 track patrol staff plan join 1,500 maintenance workers in a 24-hour strike from 18:00 GMT on Friday.
The track patrol staff are in dispute over the use of private contractors, which they say will "casualise" their jobs.
Transport for London (TfL) said it was still trying to assess the impact the walk-outs would have.
Maintenance staff are planning up to seven separate strikes up to June over what the RMT calls "lethal changes to track access".
Safety concerns
The RMT said as track workers check the network for faults and maintenance workers fix any problems, there would be concerns as to whether the Tube could run safely during the walk-out.
London Underground said safety was its top priority and more talks were planned next week.
The maintenance workers strike dates:
06:30 Friday 12 February to 06:29 on Saturday 13 February
06:30 to 18:29 on Sunday 6 March
06:30 Friday 25 March to 06:29 Saturday 26 March
06:30 Sunday 27 March to 06:29 Monday 28 March
06:30 to 18:29 on Sunday 24 April
06:30 to 18:29 on Sunday 15 May
06:30 to 18:29 on Sunday 12 June
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