Lloyds Banking Group is shifting 1,000 jobs from Scottish Widows to an outsourcer under a new agreement.
The lender said it had made a deal to move the roles, some of which are in its Clerical Medical division, to Diligenta under a so-called TUPE agreement, which means the workers' rights will be protected, with the aim of improving its heritage life, pensions and investment products.
"The new platform will make managing heritage products simpler and more efficient for LBG customers," it said.
However, Rob MacGregor, national officer at the Unite union, branded the move a "betrayal".
"The bank’s ‘sale’ of its committed... staff represents contempt for long serving and skilled employees," he said.
“The decision to simply sell off the workforce will come as a shock to staff at the bank. Unite is calling on Lloyds to reconsider this shameful deal and do the right thing by its staff who have worked hard to ensure the business is the success it is today.
“The message from Lloyds Banking Group is loud and clear and appears to be ‘so long, thanks for your efforts, you work for them now’."
Over the summer it was suggested Scottish Widows may be a takeover target for rival Aberdeen Asset Management.
Aberdeen, which finalised a £3.8bn tie-up with Standard Life last month, was the subject of speculation over a potential buyout back in July.