Staff on London Underground's Victoria Line are to stage a 24-hour strike in a dispute over workloads.
Rail, Maritime and Transport (RMT) union members will walk out from 2130 BST on 5 October.
The union claims management has gone back on an agreement to reduce the number of return journeys a driver has to make in one day.
A Transport for London spokesman said it had consulted with unions on plans to improve efficiency on the line.
The RMT said since 2003 there had been an agreement in place on the line that five rounder turns - where train operators undertake five return journeys in a day - would be minimized.
This agreement was made in recognition of the lack of air cooling systems in the drivers' cabs, the union said.
The RMT said the shifts had effectively been eliminated from the Monday to Friday rota until now.
But it claims since the opening of the new Brixton Depot, drivers have been put on five round trips, with management saying air cooling was now acceptable.
Bob Crow, general secretary of the RMT, said: "This underhand attempt to extract an extra 20% from the working day out of our members has provoked this action on the Victoria Line and we would urge the management side to re-open serious negotiations to resolve this issue."
As well as going on strike, staff will also be refusing to drive more than four round trips on the line from 30 September.
A Transport for London spokesman said: "London Underground consulted fully with unions earlier this year on plans to improve efficiency on the Victoria Line for passengers.
"A key plank of this is to look at how drivers' rosters can be organised to make the best use of staff time.
"Drivers already operate five-round trips per shift on the Victoria line at weekends, an arrangement which has never caused any problems for staff."