Freshfields is offering its equity partners up to 40 per cent of their time off as part of a package aimed at retaining female staff.
The offer comes as Clifford Chance is to vote on introducing flexible working hours for all partners.
Freshfields is introducing part-time equity partnerships this month. An added incentive for partners is the promise that those going part-time will be paid a profit share higher than the pro rata rate.
Freshfields managing partner Ian Terry says: "We are responding to an issue which particularly affects female lawyers, but the package is available to anyone at the firm.
"Law firms have difficulty in retaining female talent, who have to juggle the demands of the job with family commitments. It is tough to carry the sort of workload that everybody carries at the City."
No partners have yet taken up the offer, but Terry envisages partners taking "chunks of time" out. He anticipates that few partners will reach the 40 per cent ceiling for time out of the office.
The move is unusual among City firms, although many acknowledge the difficulty of retaining partners who dislike the long hours culture. Linklaters & Alliance already runs a part-time equity partnership scheme.
The subject has come under scrutiny after a meeting of top level firms at the Association of Partnership Practices last week. Allen & Overy partner Richard Turnor, who attended the meeting, says: "There is a serious problem that firms spend thousand of pounds training up partners and then lose them."
However, City sceptics say part-time partnerships risk weakening a firm's internal structure. They ask how partners can part-own a firm or have partial liability.
A Clifford Chance spokesman says that the firm has been investigating flexible working for some months and a report has been circulated to all partners for comment. A resolution will shortly be put to partners.