Staff at South Western Railway (SWR) have voted to hold more strikes as part of a dispute over the future of guards.
The walkouts were supported by more than four-fifths of staff who chose to vote, on a turnout of 59%.
The National Union of Rail, Maritime and Transport Workers (RMT) has not yet announced any strike dates.
SWR, which is introducing new trains that allow drivers to operate the doors instead of guards, said it was disappointed with the result.
This is the sixth time RMT members have been balloted in a dispute that has been running for two and a half years.
The most recent strike, which lasted for 27 days in December, disrupted 600,000 passenger journeys each day.
SWR is planning to introduce new trains on Waterloo to Reading services this summer, which are designed for the driver to operate the doors.
The company has promised to keep a guard “with a safety-critical role” on every train.
The RMT has previously said the change would compromise the safety of passengers.
Of 850 union members eligible to vote, 505 chose to do so. Of those, 416 voted in favour of strike action.
In a statement, SWR said it was “obviously disappointed” by the decision to strike, which it said was approved by less than half of eligible union members.
It added: “We hope the RMT will use the next few weeks as a window of opportunity to work with us to find a solution so that our passengers do not suffer more unnecessary disruption.
“Similar proposals to the ones we have made are already in place with numerous operators across London and the South East.”
SWR is the only remaining London commuter operator which has a guard operating every door on every train.
On Wednesday, Transport Secretary Grant Shapps said SWR’s franchise was “not sustainable” after the company declared a £137m loss.
Contingency measures include issuing a short-term contract to SWR’s owners or moving its operations to the Department for Transport, Mr Shapps said.
SWR’s owners, FirstGroup and MTR, were awarded the franchise in August 2017 after outbidding previous operator Stagecoach.
SWR operates routes between London Waterloo, Reading, Bristol, Exeter, Weymouth, and Portsmouth, as well as the Island Line on the Isle of Wight.