The TSB decision follows pressure from the banking union Bifu, which had threatened legal action following a European Court decision last October. The European Court ruled that failure to allow part-timers to belong to a company pension scheme constituted sexual discrimination because so many of them are women.
Thousands of the TSB's part-time staff will now be entitled to join their scheme. Their admission could be backdated as far as 1976, when the first anti-discrimination provision was agreed by the Treaty of Rome.
If other financial companies follow the TSB's lead, up to 400,000 staff working part-time in the financial sector could also benefit.
But the outlook for other part-timers, many working in the retail sector, is not so clear. Unlike their banking counterparts - who are members of non-contributory schemes - most other workers have to pay something towards their pensions.
Even if their employers do allow them to join, and the matter has yet to be tested by an industrial tribunal, they will have to make a large backdated payment into their scheme to do so.
But it may still be in their interests to join. If the amount of their employers' contributions is generous, it may pay to borrow the money and make a lump sum payment.