The agency beat previous high ranker Brunswick to the top slot, advising on ten deals last month worth nearly £6bn.
Finsbury worked on last month’s biggest deal - the sale of one of Britain’s biggest water companies, client Northumbrian Water, by French utilities giant Suez for £2.3bn.
The agency, which also handled last week’s AIM listing of Northumbrian Water, worked opposite Suez’s adviser, fledgling City PR agency M Communications.
M earns its debut Deal Watch ranking, compiled exclusively for PRWeek by corporate information database Zephyr, since the consultancy was founded last October by ex-Financial Dynamics bosses Nick Miles and Hugh Morrison.
May saw further consolidation in the water sector, with the completion of Southern Water’s £2bn sale to Southern Water Capital, a holding company created by the Royal Bank of Scotland.
Prior to its MBO from parent Chime Communications, Smithfield Financial advised RBS on the deal, gaining sixth place in the league behind M.
Meanwhile, M&A activity in retail increased last month, with bids lodged for both Debenhams and Selfridges.
Tulchan Communications, ranked fourth, advised Debenhams on its £1.5bn sale to private equity house Permira.
Finsbury advised on both deals, for Permira and investment vehicle Oxford Acquisitions on its proposed £628m takeover of Selfridges department stores.
Selfridges’ retained agency Brunswick also worked on the deal, but dropped one place to second in the May league, produced according to publicised deals involving at least one UK company.
Gavin Anderson & Co also dropped from fourth place in April, to seventh. The agency handled PR surrounding the flotation of Royal & Sun Alliance’s Australian and New Zealand operation Promina.