On behalf of 1,500 ScottishPower employees, Prospect national secretary Anne Douglas said: "While this offer is significantly above that made last year by EoN, and better reflects the value of ScottishPower, it is sad that a Scottish company like ScottishPower is likely to lose its independence.

"Prospect and the other trade unions have enjoyed a constructive relationship with the company over a long period and are pleased that the details of the bid announced include commitments to honour employees’ terms and conditions, severance terms and pension benefits for a minimum of two years.

"We are also pleased that the offer confirms that Iberdrola values the strengths of the company, including its employees, and has no plans to split it up. As a result, we expect the vast majority of our members’ jobs to be unchanged if the bid goes through. I and my colleagues will be working closely with ScottishPower management to ensure that these commitments are honoured."

Prospect, the union for energy professionals, represents 1,500 members in ScottishPower based in Scotland and the north west of England.