Commuters on the South Western Railway face further disruption after staff voted for more strikes in a long-running dispute over the use of guards.

Nearly 80% of workers who took part in the ballot held by the RMT union backed more walkouts. The union has held dozens of strikes over the last two years, including 27 days of walkouts in December, which caused travel misery for passengers.

The train operator serves about 600,000 passenger trips a day, on lines out of London Waterloo into the Surrey commuter belt and destinations including Exeter, Southampton and Bournemouth.

The transport secretary, Grant Shapps, said on Wednesday the struggling franchise was not sustainable and could be nationalised.

Commenting on the latest strike vote, a spokesman for South Western Railway said: “We are obviously disappointed with the result, although it should be noted that less than half of eligible RMT members voted for strike action. On this basis, 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, and we have already promised that we will keep guards with a safety-critical role on all our trains.”