A spokesman for the Communication Workers Union said that the union had agreed "in principle" to go on strike over the separate issue of London-weighting allowances, despite last week's ballot against industrial action.
Last week's vote saw union members reject militant calls for strike action over pay by 11,417 to 4,316.
In a statement, the Royal Mail set the scene for a clash with union members when it said that there would be no increase in London weighting. "It is more generous than the weighting that teachers, doctors and nurses get. There is no more money," it said.
Royal Mail proposes that the London-weighting allowance be increased to £2,667 a year in outer London and £3,784 in inner London, which represents an increase of £300 a year for inner London.
The CWU has rejected the offer and a spokesman said: "We want a different formula so it doesn't slip behind again."
It is unclear how bad disruption to postal services will be as the CWU has yet to give further details of the kind of industrial action its members might take.
Any strike in London will open up the market to competitors, which are allowed to carry any mail in the event of a strike. However, for this to happen the strike would have to hit distribution centres that handle London's 85m weekly letters.
The strike was condemned by watchdog Postwatch, which said the action was being taken for the sake of striking. "It looks like striking for the sake of making a point. I'm not sure what it is going to prove. Royal Mail management are not likely to give them any more money. You would have thought it was common sense for them to wait until it was doing better."
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