Furious subpostmasters are taking unprecedented industrial action over the sudden withdrawal of a penny-per-bag payment for separating first and second class letters.

The payment was worth £2.2m to the 11,500-strong post office network, and is the latest blow for postmasters who are appealing for more government contracts to keep their counters in business.

Now National Federation of SubPostmasters general secretary George Thomson has written to all post offices urging them to stop “segregating” mail with immediate effect. It is the first industrial action to be taken by the organisation in its 116-year history.

Mr Thomson said: “We are sending a clear message to Post Office Ltd that we will not sit back and watch as key payments are eroded to the point where we are starved of income and forced to close.”