Allen & Overy (A&O) and Slaughter and May have been brought in to advise FTSE 100 advertising agency WPP as its founder and CEO Sir Martin Sorrell faces allegations of misconduct.

A&O's newly appointed litigation head and employment partner Karen Seward is understood to be taking a lead role for her firm.

Separately, WPP has also appointed Wilmer Cutler Pickering Hale and Dorr to conduct an independent investigation into the allegations. It is understood that London-based litigation and investigations partner Stephen Pollard is leading the US law firm's team.

Meanwhile Quinn Emanuel Urquhart & Sullivan London co-managing partners Sue Prevezer QC and Richard East are advising Sorrell himself, alongside employment specialist firm Lewis Silkin.

The probe, first reported by The Wall Street Journal this week (3 April), relates to claims of personal misconduct and possible misuse of assets by Sorrell – allegations he has denied.

Since investing in the company in 1985, he has grown its worth to more than £15bn through a series of acquisitions, and was knighted for services to communications in 2000.

In a statement, WPP said: "The board of WPP has appointed independent counsel to conduct an investigation in response to an allegation of personal misconduct against Sir Martin Sorrell. The investigation is ongoing. The allegations do not involve amounts which are material to WPP."

A&O has a longstanding relationship with WPP, which until recently published a list of its external counsel on its website. The company's annual report and accounts for 2011 stated that its legal advisers were A&O, Freshfields Bruckhaus Deringer, Squire Sanders, Fried Frank Harris Shriver & Jacobson, offshore firm Mourant Ozannes
 and New York advertising specialist firm Davis & Gilbert.

Quinn Emanuel, A&O, Slaughters and Lewis Silkin declined to comment. WilmerHale was contacted for comment.