The Post Office is to close and franchise a further 37 of its flagship Crown offices, with the loss of 300 staff, and cut 127 financial specialist roles, workers have been told.
The Communication Workers Union said on top of the 62 branches announced for closure and franchise in 2016, the job cuts brings more "misery" to Post Office workers and the customers they serve.
General secretary Dave Ward said: "The latest round of closures is further evidence that the Post Office is in crisis and that the board of the company, backed by the Government, is simply pursuing a strategy of slash and burn.
"Today's announcement comes less than three weeks after the closure of a major government consultation on the future of the Post Office and sticks two fingers up to everyone who took part in this.
"75,000 postcards were returned to the Government signed by members of the public calling for an end to the closure and franchise programme - the Post Office and the Government have completely ignored their views."
The union has staged strikes in protest at the closure of Crown Post Offices - the larger branches usually sited on high streets.
A colourful protest was held outside the Business Department before Christmas when sackfulls of post cards from members of the public were delivered supporting the union's campaign.
Mr Ward added: "While the Government spent yesterday talking about building a shared society, today's announcement pressing on with the destruction of yet another public service shows we are reaching the point where we will have little left to share.
"The CWU will not accept this and we will be stepping up our political and industrial campaign to fight for the future of the Post Office."
Deputy General Secretary Terry Pullinger said: "The arrogance of the Post Office and Government is stunning. The Post Office network has been reduced by more than 50% over the past 30 years and continues to be run down. It requires a proper business plan for growth and investment which is being ignored.
"Its IT platform is being held together with sticky tape and it continues to outsource and close its outlets, yet they dress it up as a success and honour its directors.
"A con is a con regardless of how it's dressed up and the British public and our members are being conned on a grand scale into believing that a Post Office without modern new products and services, and the right investment can survive.
"Shame on them and have no doubt our loyal members who have served the British public most of their working lives have every right to protest and we will continue to support and lead them."