Graham Martin, the general counsel (GC) of Tullow Oil is to step down as legal chief in the New Year, as part of a re-shuffle at the top of the energy giant's legal team.

Martin will relinquish his duties as GC with effect from 1 January 2014, but will continue to serve on the board as an executive director and company secretary.

He will be replaced in the top legal role by Peter Sloan, who has worked as deputy general counsel since 2011.

Before joining the FTSE 100-constituent in 1997 as a legal and commercial director, Martin practiced at both Vinson & Elkins and Dickson Minto, where he served as the principal external legal adviser to the company. He moved into the company secretary position in 2008.

With operations in 25 countries, London-headquartered Tullow last year generated $2.3bn revenues

In the UK, Tullow has historically used a range of firms including Norton Rose Fulbright, Ashurst and Freshfields Bruckhaus Deringer.

Earlier this year, Ashurst partner Richard Mott and One Essex Court's David Wolfson QC advised Tullow on its successful tax claim in the High Court against Heritage Oil and Gas Ltd & Anor.

During the case, Martin was cross-examined for three and a half days, and was singled out by Mr Justice Burton as an "impressive and honourable witness", though the judge reprimanded the GC for disposing several documents from his office in Kampala that were relevant to the case.

In 2011, the company turned to Vinson partner and London head Alex Msimang to advised on the $2.9bn (£1.8bn) sale of stakes in three Ugandan oil exploration projects to French energy group Total.