The split comes after Finsbury decided this week to represent Taylor Woodrow for its proposed merger with George Wimpey - a deal that could create Britain's largest homemaker.
The Persimmon account, believed to be worth more than £100,000, could see M: go up against Finsbury - if Persimmon launches an attempt to scupper the Taylor Woodrow and Wimpey deal. City watchers have suggested a move for Taylor Woodrow or another large housebuilder is likely.
Persimmon has brought in an M: team headed by partner Ed Orlebar. Orlebar had worked on the Persimmon account before he left Finsbury for M: in May last year (PRWeek, 12 May 2006).
M: declined to comment. Finsbury did not return calls.
The TW/Wimpey deal is the biggest to date in Britain's building sector. Wimpey is using Weber Shandwick for public affairs and media relations.