Last year, the Spanish-owned company hired contractor Accenture to take over the IT services from another company, SAIC. At the time, Prospect and Unison, the unions representing these workers, protested over plans to offshore some of the work to India. A deal was negotiated to keep this work in the UK.

But just two weeks ago, ScottishPower changed its mind again, withdrawing from the Accenture arrangement and re-awarding the contract to yet another private company – IBM.

More than 300 workers at sites in North Wales and East Kilbride are affected, including some Prospect software and IT support engineers.

Prospect negotiator Malcolm Currie said: “Members are angry and disappointed at the sudden change of plan, which took place without proper consultation. That means tomorrow these workers will start work for a new contractor without knowing their rights relating to pay and other terms and conditions, including their pensions.

“IBM has agreed to an eight-week consultation period post transfer, but this does not satisfy the legal obligations to consult before any transfer.

“The speed of the switch also presents enormous practical problems, in terms of day-to-day operations and the proper transfer of employee data.

“This is a terrible way to treat committed staff who have already been through more than a year of uncertainty over their jobs, yet have continued to deliver professionally. Workers should have properly consulted, but ScottishPower and IBM haven’t even given them that courtesy. It seems that saving money is more important.”