Employees at train operator South Eastern Trains are to be balloted on strike action in a row about jobs and working conditions.
Members of the Rail Maritime and Transport Union will vote in the next few weeks on whether to launch a campaign of industrial action.
The union has been protesting at plans to reprivatise the company, which runs services into London from Kent and other parts of the South-East.
Officials argue that services have improved since the franchise was taken away from Connex two years ago.
But the government is pressing ahead with plans to re-privatise the service even though the union has warned of possible job cuts and attacks on working conditions.
RMT general secretary, Bob Crow, said he had asked the company to give assurances there would be no compulsory redundancies or worsening of pensions and other conditions through the reprivatisation.
He said: “We made it quite clear that failure to agree to do so by the end of August would result in a dispute between us and we have now set in motion a ballot for strike action among our 1,350 members.
South Eastern said a meeting had been set up with the RMT on 20 September to discuss its demands.