Tube drivers in London are to hold a 52-hour strike in a row over working conditions and staffing.
Rail, Maritime and Transport workers' union (RMT) members on the Piccadilly Line will walk out between 21:00 BST on 11 July and 01:00 on 14 July.
The RMT said there had been "repeated problems" on the line including not enough drivers and an "ageing fleet".
Transport for London (TfL) urged the union to work "constructively" with it on the issues.
The walk out will coincide with Donald Trump's visit to the UK on 13 July.
About half a million passengers travel on the Piccadilly Line each day, with the route linking central London to Heathrow Airport.
RMT general secretary Mick Cash said the union "had no option but to put on these strike dates" because of "the failure of LU management to get a grip".
"It is now down to LU bosses to start listening to their members, [and] take the raft of issues at the heart of this dispute seriously," he said.
"We urge the RMT leadership to work with us constructively on the local issues they have raised," Nigel Holness, director of network operations for London Underground (LU) said.