It comes on top of the 62 branches announced for closure and franchise in 2016, with the job cuts bringing more "misery" to Post Office workers and the customers they serve, said the union.
The branches ear-marked for closure or franchise are so-called Crown offices, which are larger and directly-managed by the Post Office - as oppose to franchises.
The branches closing are: Stamford Hill (London), Leyton (London), Church Road (London), Stoke Newington (London), Walthamstow Central (London), Harringay (London), New Cross Gate (London), South Kensington Station (London), South Woodford (London), Abbey Wood (London), Harlesden (London), Hornchurch (London).
The branches provide vital banking and services to older and vulerable people, such as pension and benefit payments.
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."
Roger Gale, sales and trade marketing director at the Post Office, said: “We’re committed to maintaining the Post Office’s special place on the high street and the changes we are making underpin our continued commitment to give communities in every part of the country access to essential services.
“The Post Office’s network of more than 11,600 branches is easily the largest in the UK, with 17 million customer visits a week. The vast majority of these branches are run with partners, and in the locations announced today we believe this will also be a more sustainable approach for the long term. With consumer habits changing, and the high cost of maintaining premises in prime high street locations, franchising helps us to keep services where our customers want and need them.
“We will take time to identify the right partners over the coming months and all proposals will be subject to local consultation.
“Post Office has a strong record of supporting people through change and we will be keeping affected staff fully informed as we develop our plans.”