TSB will close 164 of its branches and cut 960 jobs, blaming "a significant shift in customer behaviour" as more customers bank online.
The figure is in addition to the 82 branches it said it would close in November, when it set out plans to save £100m by 2022.
A list of the affected branches and their closure dates has now been published after staff were informed.
Union Unite described the move as "a dark day for the finance sector".
The bank said the closures were not an "easy decision" and had been accelerated by the pandemic.
From the end of next year TSB will have 290 branches - down from 475 currently.
The bank hopes most of the job cuts will come through voluntary redundancies, and said it would also create 120 new positions.
It said branches with the lowest footfall would go.
"Our customers are banking differently - with a marked shift to digital banking," said boss Debbie Crosbie.
"This means having the right balance between branches on the high street and our digital platforms, enabling us to offer the very best experience for our personal and business customers across the UK."
Unite's national officer Dominic Hook said: "Unite has argued for some time that the financial services industry has a social responsibility not to walk away from its local customers who continue to need access to banking in bank branches.
"It beggars belief that just seven years ago TSB had 631 local branches and this announcement will reduce that number to merely 290 branches. "
Today, TSB has 475 branches. By the end of 2021, it will have 290.
They are stark numbers for a bank that has always promoted itself as a community service, and once attacked rivals for cutting branches.
The closure plan will be a blow to staff, aware of the wider economic outlook as they consider whether to volunteer for redundancy.
It will also be a blow to customers who still like to use a branch, and to small businesses that hope to keep travel times to a minimum when depositing cash.
Partnerships with the Post Office and cash collection services aim to mitigate these problems.
But, while customer behaviour is changing across the bank sector, some people are being forced to bank in a different way - and they may not feel comfortable doing so.
TSB is the latest bank to announce closures during the pandemic.
In August the Co-operative Bank has said it will cut 350 jobs and close 18 branches due to the current "economic uncertainty" and the shift to online banking.
Natwest Group also said it would cut 550 jobs in branches and close one of its remaining offices in London.
TSB, which is owned by Spanish lender Banco Sabadell, has been dogged by technical problems, with an IT failure in 2018 leaving up to 1.9 million customers unable to bank online for several weeks.
Customers were moved on to a new system, but an investigation found it had not been tested properly before going live. It cost TSB a total of £330m for customer compensation, fraud losses and other expenses.
As recently as last month users were unable to access online banking. And last year, a "processing error" meant wages and other payments were not paid into some TSB customers' accounts.