Campaigners fear that the government is preparing to hand the running of a bank account used by four million pensioners and other claimants to a private firm.
This move could see a further 3,000 post offices close.
The future of the Post Office Card Account was due to have been announced by ministers earlier this year, but has been repeatedly delayed.
Many people suspect that the contract will be handed to Paypoint, a private firm.
The decision is thought to have been further delayed by the Glenrothes by-election, which was won by Labour last week, since two thirds of Scottish post offices may close if Royal Mail loses the card account contract.
CWU Milton Keynes rally
The postal workers’ CWU union is inviting trade unionists from across Britain to rally in Milton Keynes this Saturday 15 November in opposition to Royal Mail’s plans to close scores of mail centres.
Assemble at 1.30pm for march from the threatened mail centre in the town at Dawson Road to a city centre rally.
For further details about the campaign and protest email email@example.com