TSB is considering closing up to 100 branches, according to a trade union source, in a move that could put 400 jobs at risk.

The challenger bank has been working to repair its reputation following its IT meltdown last year but is looking for around £100m in cost savings, with its 544-strong branch network under scrutiny.

Mark Brown, the general secretary for the Affinity trade union said the lender is likely to unveil up to 100 branch closures when it unveils its new strategy on 25 November. Affinity, while not officially recognised by TSB, represents around 3,900 of its 7,795 staff.

Most of the branches in question are likely to employ just a handful of workers, and Brown estimated that around 400 may be affected by the moves. TSB in the past has tried to redeploy staff when possible. Branch closures are a common occurrence in high street banking and more than a third of the UK’s bank branches have shut for good in less than five years, with more than 3,000 closures since 2015.

TSB’s parent company, Spanish lender Sabadell, appointed Debbie Crosbie as TSB chief executive last year following an IT failure which locked millions of customers out of their accounts and led to the departure of Crosbie’s predeccesor, Paul Pester.

Brown said: “The results of TSB’s strategic review are going to be more branch closures and more job losses right across the bank. Hundreds of staff who saved TSB following its IT meltdown last year are going to be sacrificed on the altars of costs, efficiency and technology.

“It’s clear Ms Crosbie was brought in by Sabadell to cut costs, increase income and sort out the IT system before TSB is sold to the highest bidder in a few years time.”

Many of the closures are expected to come from the 94 branches which are now operating under reduced hours, or are only open once or twice a week following controversial changes introduced over the summer. The remaining closures may come from the network of Cheltenham & Gloucester Building Society branches which were originally slated for the axe by Lloyds – the former owner of TSB – as early as 2009.

TSB’s new chief operation officer Suresh Viswanathan told staff last month that the lender spends around £180m each year on operating costs such as IT systems and staff. In a transcript seen by the Guardian, the executive suggested that the number should be nearly £100m lower.

Ged Nichols, general secretary for TSB’s official union Accord, would not comment on the figures around job cuts or branch closures, but said: “If TSB is going to overcome its challenge that has implications for partners in the business and we would want and expect the bank to be true to its values and culture and do things in the right way in consultation with the unions.”

A TSB spokeswoman said: “We don’t comment on speculation.”