Prospect negotiations officer Ian Perth said, ''We met with Scottish Power on Monday and many of our initial concerns were addressed, however, there are significant outstanding issues such as an annual pay award, which is 10 months overdue, and backdated overtime issues that need to be clarified. ''

There are four recognised unions within Scottish Power. All unions have unanimously called on the employer to resolve these long standing issues before any takeover is finalised.

''In recent months, Scottish Power has made no secret of their willingness to sell the generation assets to the right bidder,  so our members are not very surprised by the announcement," added Perth.

"We understand the takeover will be completed by share transfer, so technically some might see this as a 're-branding', however many of our members have proudly worked under the banner of Scottish Power and its structures for decades, and this change won't be taken lightly.

"We also welcome Scottish Power's announcement that it will maintain its pledged £5.5 billion of investment in Scotland and the UK to 2022.

"We will begin a series of meetings with the employer from 23 October and hope that our members will have certainty as soon possible."

Mike MacDonald, Prospect Negotiator with responsibility for Drax said: "Prospect has a thriving branch in Drax, and through the handover process we will be liaising with our colleagues to ensure appropriate arrangements are made."