Barry O’Brien, a former head of corporate finance at Freshfields Bruckhaus Deringer, has been fined £9,000 and has agreed to pay a further £50,000 in costs over his conflicted role in Philip Green’s aborted takeover of Marks & Spencer.

The Solicitors Disciplinary Tribunal announced the fine this afternoon after it emerged that Mr O’Brien, one of the most respected corporate lawyers in the City of London, would not contest the Law Society's allegations against him.

He was fined £5,000 for breaching his duty to a client and £4,000 for bringing his profession into disrepute.

Mr O’Brien, who has retired from Freshfields' partnership but remains employed by the firm as a consultant, volunteered to pay a further £50,000 to cover the Law Society’s costs in bringing the case.

The case centred on Mr O’Brien’s decision to advise a consortium run by Philip Green on a proposed £9 billion takeover of Marks & Spencer, a longstanding Freshfields client, in 2004.

Mr O’Brien admitted that accepting the mandate was an error of judgment for which he and the firm “apologised unreservedly”.

Mr O’Brien and Freshfields were subsequently banned from advising Mr Green’s consortium after the High Court agreed with M&S that there was a conflict of interest because of the firm’s relationship with the retailer.

Mr O’Brien was referred to the Solicitors Disciplinary Tribunal following an investigation by the Law Society, the profession’s governing body.

In a statement, Freshfields said: “Barry O’Brien has reached an agreement with the Solicitors Regulation Authority (SRA) to end the long-running investigation into the firm’s acceptance of instructions in early 2004 to advise [Philip Green’s] Revival Consortium on its possible offer for Marks and Spencer.

“As part of the agreement, Mr O’Brien did not contest the proceedings in front of the Solicitors Disciplinary Tribunal today and accepted a fine from the Tribunal.”

Guy Morton, Freshfields Bruckhaus Deringer’s joint senior partner said: “We are grateful to Barry for volunteering, in the best interests of the firm, to bring these protracted proceedings to a close by not contesting the matter further. We very much regret that a bona fide and collective decision made by partners on behalf of the firm led to disciplinary proceedings.

“We are relieved that this entire matter is closed and that Barry can now fully concentrate on continuing his outstanding career.”

The Tribunal was told how Mr O'Brien was one of the leading corporate lawyers of his generation who was also active in several charities. His error of judgment was described as a one-time blip on an otherwise distinguished career that included masterminding a rescue plan for Lloyd's of London that was said to have saved the insurance market from collapse.

In recognition of his contribution, Mr O'Brien was awarded Lloyd's Silver Medal, a high honour typically reserved for insurance brokers and which has not been presented to a lawyer since 1824.

The Tribunal also heard praise for Mr O'Brien from two established legal directories that described him as "the first choice, every time, for complex advice", a "robust heavyweight" and "first class adviser".

Mr O'Brien's legal team reminded the Tribunal that neither M&S nor the consortium had complained about the incident and that neither had suffered any loss as a result of Freshfields' brief involvement in the bid.

Freshfields, one of the largest law firms in the world, is awaiting the outcome of an unrelated case at an Employment Tribunal in which a former partner has accused the firm of age discrimination.

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