Mishcon de Reya may have to start charging for its work on the Diana, Princess of Wales Memorial Fund, which has up to now been carried out on a pro bono basis.
It is understood that because of the enormous amount of work potentially involved, the firm is likely to charge for its professional services once a full complement of trustees is established.
The first two trustees, Mishcons partner Anthony Julius and the Princess's private secretary at Kensington Palace, Michael Gibbins, have worked in close consultation with the Charities Commission to draft the trust deed.
The princess's eldest sister, Lady Sarah McCorquodale, also agreed to become a trustee last week. Mishcons aims to have about eight trustees.
A clause has been included in the deed which allows Mishcons to charge for any work it does for the trust in a professional capacity.
Mishcons partner Jonathan Cameron, head of media and multimedia, has already been responsible for registering a Diana logo and the name Diana, Princess of Wales, as trademarks in the US and Europe to stop the sale of merchandise that uses her image.
The probate work is being led by Christopher Allen, the head of the firm's private client department.
Mishcons said last week it had processed £8m for the fund but this did not include what its tax lawyers may be able to recover, nor the expected £100m from sales of the Elton John record, Candle in the Wind.