The Communication Workers Union is claiming that the 14.5% pay deal offered to staff has "more strings than the Philharmonic Orchestra" and is set to ballot its 200,000 members on whether to hold the first national postal strike since 1996.
Leighton upset unions last month when he wrote directly to staff, offering them the 14.5% pay rise that would bring the minimum wage to £300 a week.
The union reacted by writing to members informing them that the deal offered by Royal Mail amounted to only 4.5% in a bid to encourage them to vote against the pay deal.
A vote against the deal is almost certain to lead to industrial action following the CWU's recent pronouncement that any strike mandate would be followed by industrial action.
Dave Ward, the union's postal deputy general secretary, said: "Specifically, we would have to accept the loss of 30,000 jobs in the industry. We will not do this. It would be signing the death warrant for the entire postal service. It would condemn the Royal Mail to massive and terminal decline."
Ward says he has written to the company today pointing out that the ballot decision would not be implemented for between a week and ten days.
"This gives Royal Mail time to reconsider what they have offered us. If they are prepared to do so, the CWU is always ready to negotiate," Ward said.
Leighton is expected to write to staff again, urging them to vote 'no' to strike action.
The relationship between Leighton and the union became increasingly strained after union literature accused the chairman of awarding himself a fat-cat pay increase.
Leighton's lawyers say the bonus figure it quotes is incorrect and are demanding that the record be put straight.
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