It includes among its customers a list of well-known charities including Age Concern, the BBC's Children in Need Appeal, Childline, Shelter and the British Red Cross Society.
Farrer & Co maintained second place again, with a total of 27 clients, up from last year's figure of 22. Its business includes work for the Prince's Trust and the Institute of Cancer Trust.
Third-ranked was Turner Kenneth Brown, adviser to the Royal National Lifeboat Institution, the National Council for Voluntary Organisations and the World Association of Girl Guides and Girl Scouts, with 23 accounts.
Eighteen clients earned fourth place for Linklaters & Paines, which advises Cafod, and fifth place was shared by four firms including Allen & Overy, which acts for the Prince's Youth Business Trust, and Harbottle & Lewis for the Terence Higgins Trust.
Judith Hill, a committee member at the Charity Law Association and chair of Farrer & Co's charity group, says voluntary sector contracts were becoming more important for many firms.
"Charities are an important part of the scene and will continue to be so," she says.
"For our firm it has always been important. We have acted for charities for the best part of our 300-odd years, but I have noticed that a lot of firms have recently come into charity work.
"During the recession when commercial clients weren't doing anything, firms looked around for other kinds of clients and found charities."
But Andrew Phillips, senior partner at Bates Wells & Braithwaite, casts doubt on the value of the league table.
He says: "I know we came top but these tables do not reflect the ability of firms to meet the needs of their clients."