Thousands more part-time workers were due to benefit after Barclays said it was planning to back-date pension benefits.

The High Street bank said it was in the process of writing to 13,500 former employees, most of whom were women, telling them they would be given the same pension rights as full-time staff, dating back to April 1976.

The move came after HSBC said it was back-dating pension benefits for part-time workers for employment between 1976 and 1992, when they were allowed to join its scheme.

HSBC estimated up to 3,500 staff could benefit from the back-payments, which it said would cost it millions of pounds.

Barclays refused to put a figure on the cost of its move but, given the large numbers of people involved, it is likely to cost the bank considerably more than HSBC will have to spend.

Barclays has written to all part-time staff who were employed between 1986 and 1990, when part-timers were given the same pension rights as full-time staff, but is calling on people it employed before 1986 to contact it themselves.

Final details of the settlement are still being finalised with banking and finance trade union Unifi.

The decision by both banks to back-date pension membership follows a House of Lords ruling in February last year which gave part-time employees the right to join pension schemes retrospectively.

Unifi, which had been negotiating with both HSBC and Barclays, welcomed the move.

Union officer Linda Gregory said: "We have been negotiating with Barclays for sometime and they have made contact with people who stand to benefit.

"Hopefully it will form a template for other employers."