Karen Clayton, the UK general counsel and company secretary of National Grid has left the company halfway through an ongoing review of the energy giant's external legal advisers.

National Grid's group general counsel Alison Kay has assumed responsibility for the review and Clayton's other responsibilities on an interim basis, while the FTSE 100 company searches for a full time replacement.

Legal Week understands Clayton (pictured) had initially planned to exit the post at the end of this month, but stood down from the role in mid-September, after agreeing leaving terms with the company.

Regular law firm advisers to National Grid were told of Clayton's departure in the last few weeks.

Clayton trained at legacy Herbert Smith before stints at legacy Mallesons Stephen Jaques in Melbourne and Osborne Clarke, before joining National Grid in 2003.

At the time of her departure, she was mid-way through a review of the company's legal advisers, which is set to come into effect on 1 April 2015.

National Grid's panel currently comprises Allen & Overy and Linklaters as the company's main corporate advisers, while Berwin Leighton Paisner, CMS Cameron McKenna and Eversheds all hold spots to advise on commercial, construction and disputes work.

The remaining firms include Irwin Mitchell, SGH Martineau, Dentons, Walker Morris, Squire Patton Boggs, Bircham Dyson Bell, Brook Street des Roches, Fieldfisher, DLA Piper and Needham & James.

Despite being one of the largest listed companies in the UK, National Grid has a fairly modest annual legal spend of between £15m and £20m.

However, it is a figure Clayton said she was keen to bring down further, when she spoke to Legal Week earlier this year.

"Like most clients, we have a whole range of cost and fee structures for different types of work, but litigation spend tends to be more on an hourly rate basis," she said. "It has been that way forever and a day, and I'm keen to move away from it."

Irwin Mitchell, which was appointed to National Grid's 2011 panel for debt recovery work, lost its place in March following a mini-review of advisers

In a statment, National Grid said: "We completely understand her decision and she left with our best wishes and thanks for all her hard work over the years. Our panel review is still at a relatively early stage and we will be taking that forward."