Trade unions are planning to press supermarket chain Sainsbury's for compensation for 900 IT staff transferring from the supermarket to Andersen Consulting.
The £1.5m deal with Andersen will allow the supermarket chain to cut costs and break free of its legacy systems, which consume £200m a year.
But the MSF Technology Professionals Association said the move would mean that IT workers lose out on the pension and share option benefits they enjoy at Sainsbury's.
"There is quite a difference between the Sainsbury's pension scheme, which is final salary, and the Andersen scheme, which is money purchase," said Peter Skyte, national organiser of the compensation claim.
Staff will also lose out because the Andersen scheme is less generous than the Sainsbury's scheme.
The retirement age at Andersen is also five years earlier than Sainsbury's, leaving less time to build up their funds, said Skyte.
Sainsbury's said it would comply with the Transfer of Undertakings Protection of Employment regulations, which guarantee employees equivalent terms when they transfer to a new employer.
"The elements of the remuneration package might vary but overall they will be as well, or better off," claimed Sainsbury's.
IT staff will have more opportunities to develop their careers at Andersen, the retailer added.