The disagreement over pay at bus company Thamesway is set to continue - and could go to a fifth ballot.
The revelation comes after the "final" pay offer was rejected by a margin of just 13.
The latest offer would have seen rates rise from £4.71 an hour to £5.35, adding more than £20 to pay packets each week.
But Bill Lumb, Transport and General Workers Union regional organiser, said he would rather let another 50 votes take place if it meant getting a suitable result.
He said: "Both sides are now assessing the situation, bearing in mind it was such a close vote. There could be a further move but that is yet to be confirmed. If there is it could mean a fifth ballot.
"If there is no other proposal the arbitration service ACAS will step in and take over. We would prefer to settle this ourselves through a democratic ballot. In the long term that will be better for industrial relations."
The latest round of ballots saw 124 bus workers voting to accept the offer and 137 rejecting it.
The dispute over pay at Thamesway came to a head in October when workers rejected a 5.3 per cent increase and voted for strike action - which was averted by the promise of further talks at ACAS.
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