Simmons & Simmons has changed its partnership agreement to include gardening leave for the first time after losing nine partners this year.
Four of the high-level departures took place in the last three months.
A source close to the City firm says: "Simmons has just put in place a new partnership agreement which requires six-months' notice of retirement.
"It also enables the senior partner, at his discretion, if a partner is going to a competing firm, to put that partner on six-months gardening leave."
The first partner to be affected by the new covenant is capital markets specialist Colin Mercer, who is leaving to join Brown & Wood.
He has already left the firm and will join the London operation of the US firm on 11 May 2000 (see box).
He will join former Simmons partner John Russell, who went to Brown & Wood last month (The Lawyer, 8 November).
Russell joined Brown & Wood only a week after handing in his resignation at Simmons. But he is restricted under his former partnership agreement from soliciting clients.
A senior lawyer with close ties to Simmons says: "My own view is that you only send someone on gardening leave if you are really concerned they are going to damage the firm's business.
"It is the fact that Colin is going to Brown & Wood that has really panicked them."
A Simmons spokesman confirms the partnership deed has been altered to include gardening leave but refuses to be drawn on whether it was a direct result of this year's high staff turnover.
But he points out that as well as losing a raft of partners, the firm has also made a series of lateral hires over the last few months.
Simmons equity partner Gordon Stewart was poached by Richards Butler in October. Senior telecoms partner Chris Watson is serving a notice period until he joins Allen & Overy in the new year.
In August this year, long-standing corporate partner Richard Allnutt left the firm to go in-house as group counsel with global information technology services supplier ICL.
Increasing numbers of law firms are beginning to introduce gardening leave, where partners are not permitted to work although they still receive their salary, in response to an increasingly volatile market.