Rail commuters on some of the country's busiest routes are facing disruption due to a planned five-day strike over the future of train guards.
Members of the Rail, Maritime and Transport union (RMT) plan to walk out on Tuesday over South Western Railway's "failure" to rule out it would move to a driver-controlled operation.
London commuters and racegoers at Royal Ascot have been advised to check details of trains online.
SWR called the strikes "unnecessary".
It said while services may be disrupted by the strike, there will still be trains running, and advised commuters to plan in advance and check departure times on its website.
Planned industrial action was suspended in February as a resolution seemed in sight, with the RMT claiming SWR had pledged "each passenger train shall operate with a guard with safety critical competencies".
But RMT said SWR had now "rowed back" on its public pledges as it refused to rule out future driver controlled operations - which would see the role of the guard "carved up completely".
RMT general secretary Mick Cash said members were "angry and frustrated" as they had suspended action in "good faith" only for SWR to "fail to bolt down an agreement that matches up to our expectations on the guard guarantee".
He also criticised SWR's "insistence" that future schemes would be "governed" by the protection of company profits rather than that of "the travelling public".
A SWR spokesman said it was "very disappointing" the union had decided to call the strike despite dates being set for more talks.
"Clearly, they have decided to target popular events such as Royal Ascot with this cynical action which is driven by internal RMT politics," the spokesman said.
The company said it met with union representatives last week to fix new dates for talks but the unions were "insistent on going ahead with their unnecessary strike".
It said it had matched RMT's request to keep a guard on each train and wanted to move on to discuss how to make the most of new technology on board.
The spokesman said the company "remains committed to finding a solution".
Passengers heading to Twickenham, Hampton Court and Royal Ascot, have been advised to allow extra time for their travel.
The Royal Ascot event runs for five days from Tuesday.
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