Train drivers from the Aslef union have rejected the deal agreed by their leadership and Southern rail in a major setback that could lead to more crippling strikes on the network.
Many believed the dispute regarding driver-only operated (DOO) trains had been settled on 2 February when Aslef chiefs and Southern bosses emerged from TUC negotiations with a deal to put to Aslef members.
But on 16 February 54.1% of Aslef drivers rejected the deal from a turnout of 72.7% meaning that their representatives will be forced to re-enter peace talks with Southern.
Some 953 Aslef members were asked if they agreed to compromises reached with Southern about the increased use of DOO trains on the line, which sees train drivers solely responsible for operating train doors.
Of the 693 that returned their papers, 374 said they did not agree, 317 agreed while two ballots were returned invalid.
Mick Whelan, general secretary of Aslef, the train drivers' union, said: "We understand and support the decision arrived at democratically by our members and will now work to deliver a resolution in line with their expectations."
Nick Brown, chief operating officer of GTR said: "Naturally we're saddened and hugely disappointed, as will be our passengers, with today's decision by drivers, particularly as the agreement carried the full support and recommendation of the Aslef leadership."
The RMT, who represent on-board train staff, had accused Aslef of a "shocking betrayal" when they appeared to have struck a deal with Southern earlier this month. The RMT leadership has not resolved its dispute with Southern and its members plan to strike again on Wednesday 22 February.
Walkouts by on-board staff have proved far less debilitating than those by drivers. Southern are able to operate around three quarters of services when RMT members down tools compared with complete shutdowns when drivers take industrial action.
Members of both unions claim that the move to DOO trains puts passenger safety at risk while Southern point to a report by the rail watchdog that gave them the green light.
RMT general secretary Mick Cash said: "RMT repeats the call to Southern to give the guarantee of a second, safety critical member of staff on their trains and to sit down with the unions in new talks around the issue of safe train dispatch."