A five-day strike by South Western Railway (SWR) staff has started in the latest industrial action over the role of guards on trains.
Members of the Rail, Maritime and Transport union (RMT) began the walkout at 00:01 BST.
RMT said staff had been left with “no choice” but to strike, and accused SWR of failing to guarantee guard roles.
SWR said the action was “unnecessary” and it was “cynical” of the union to target events such as Royal Ascot.
The rail company runs services in London and Berkshire, Surrey, Hampshire, Dorset, Devon, the Isle of Wight and Somerset.
A reduced service will be running across the network, but the rail company said extra morning and early evening trains would run between London Waterloo and Reading to help people heading to Royal Ascot.
The long-running dispute has resulted in 29 days of industrial action by RMT members over the past two years.
A planned walkout in February was suspended after the RMT said there had been “substantial progress” in talks with the company.
This followed the suspension of strikes on Northern Rail after the company agreed to guarantee a conductor on all trains.
However, the RMT said SWR had now “rowed back” and was refusing to rule out future driver-controlled operations.
The union said this would see the role of guards “carved up completely”.
RMT general secretary Mick Cash said the company had “dragged their heels and failed to bolt down an agreement that matches up to our expectations on the guard guarantee”.
“The company has refused to give assurances on the future operational role of the guard, fuelling fears amongst our members of a stitch-up,” he said.
SWR said the company met union representatives last week to arrange fresh talks but the union was “insistent on going ahead” with the action.
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.
An SWR spokesman said the company “remains committed to finding a solution”.
Passengers heading to events at Twickenham, Hampton Court and Royal Ascot have been advised to allow extra time for their travel.
The action is scheduled to end at 23:59 on Saturday.