Aslef is considering its next move, with seven days’ notice needed for a strike. Photograph: Philip Toscano/PA

Drivers on Southern rail trains have voted overwhelmingly to go on strike in a dispute over the operation of train doors, the union Aslef has said.

The ballot for industrial action is likely to spell more serious disruption on Britain’s biggest commuter franchise. Conductors in the RMT union have held a series of strikes this year, with more walkouts planned for next week and the Christmas period.

The dispute is over the extension of driver-only operation on Southern trains, a move which sees conductors renamed as onboard supervisors, giving the driver responsibility for closing the doors. Unions say there are safety risks, a claim denied by Southern and rail industry bodies.

Aslef held a ballot to strike earlier this year, but it was ruled unlawful in a high court challenge brought by the company.

In the latest ballot, 87% of drivers backed a strike. The turnout was 77%. Aslef is now considering its next move, with seven days’ notice needed for a strike.

It could coincide action with the RMT’s pre-Christmas walkout, severely affecting the ability of Southern to run any sort of service. The company has operated about two-thirds of trains during recent strikes, but the franchise has been plagued by problems ranging from understaffing to delays caused by engineering works.

Charles Horton, the chief executive of Southern’s operating company, Govia Thameslink Railway, said: “We believe this ballot was wholly unnecessary and unjustified in the first place and we’re disappointed that the union is now contemplating industrial action.

“It’s perfectly safe for the driver to have sole responsibility for the operation of a modern train, and that’s how a third of the trains up and down the country – with the full agreement and support of Aslef – already operate today.”

Meanwhile, Southern is investigating a fire that broke out on a train on Monday morning shortly before rush hour. The fire, discovered at 5am in an empty carriage at Eastbourne, caused delays and cancellations in the area.