Unionist leaders in Northern Ireland have written to Taoiseach Brian Cowen to formally object to the ESB's planned takeover of Northern Ireland Electricity.

Last July, the ESB announced plans to buy NIE, which controls Northern Ireland's electricity network, from its owner, Viridian, for €1.25bn.

In a rare joint approach, the DUP and the UUP have come together to formally object to the planned takeover.

It has emerged that Northern Ireland First Minister Peter Robinson and outgoing UUP leader Reg Empey have written to Mr Cowen, claiming that the deal has major political implications.

They described the plan as ‘inappropriate’ and said it amounted to the purchase of a ‘key component’ of Northern Ireland’s national infrastructure.

They also said there would have been ‘political outcry’ had the Northern Ireland Executive or the UK government sought to acquire the Irish electricity grid.

Sinn Féin has criticised the move and accused the unionist leaders of engaging in the ‘politics of a bygone era’.

A spokesperson for the ESB said the company's plan to purchase NIE remained on course and was expected to be completed by the end of the year.