Mayer Brown is suing client Vantis for unpaid fees, claiming more than £400,000 plus interest.
The firm has a longstanding relationship with the accountancy and professional advisory group, with partner Stephen Bottomley advising on its AIM flotation in 2002.
Bottomley advised the company on its 2005 acquisition of Numerica, which was also listed on AIM, but Mayer Brown claims not to have received payment for the advice.
In addition, the firm is claiming unpaid fees in relation to advice given on the disposal of Numerica's Bristol, Manchester and Southampton offices as well, as on the acquisition of AV Audit's shares.
Mayer Brown issued three invoices in total between October 2005 and December 2006, for £75,000, £428,842, and £2,890, but claims to have been paid just £75,000.
In a High Court claim the firm's adviser, Beachcroft partner Marcus Thomson, said Vantis owes Mayer Brown £431,532.47, adding that his client is claiming interest on the sum for the period between 27 January this year and the judgment date. With interest charged at a rate of 8 per cent per annum, at the time of filing the claim the additional sum came to £20,429.28.
Mayer Brown declined to comment on the case. Vantis was unavailable for comment.