Staff in Transport for London's travel information and call centres will walk out for 24 hours in a dispute over a bonus for the Olympics, a union said.
The Rail, Maritime and Transport (RMT) union said its members will go on strike from 21:30 BST on 1 July.
Staff were not permitted to volunteer as 2012 travel ambassadors and some are banned from taking leave, the RMT said.
TfL said the action was "unjustified" as 14% of the 212 non-frontline staff balloted by the union voted to strike.
– Tricia Riley, TfL's HR director
The RMT leadership's decision to call a strike is completely unjustified given just 14% of their 212 members voted for strike action.
"They are clearly out of touch with their members and should call off their planned action immediately.
"These are mainly TfL contact centre roles and no staff are being asked to change their day jobs or have been expected to change their working patterns without consent.
"We do not expect this to have a significant impact on the service we provide to our passengers."
The union said TfL had refused to meet at the conciliation service Acas to resolve the dispute.
– RMT General Secretary Bob Crow
"These TfL staff will be as busy as anybody else and the attempt to bully them into bans on annual leave and unilateral changes to working conditions for nothing in return is a disgrace