Top-four City VAT partner Stephen Coleclough is leaving Simmons & Simmons to join Coopers & Lybrand because of the "ridiculous hours" he had to work.
Coleclough, who took over from Edward Troup as head of Simmons' tax group 18 months ago, said he ended up taking on part of Troup's direct tax practice as well as VAT and running the group.
"I had too much work," said Coleclough. "No matter how hard you try to manage the clients or your colleagues you always end up doing ridiculous hours."
"I had two days off in the first six months and there hasn't been a night when I haven't come home before midnight. I was working about 80 hours a week, week in, week out."
He added: "At some point you have to stop and think 'let's go somewhere where they might let you have weekends off.'"
Coleclough said Coopers considered that if you worked more than 50 hours a week you could not be working at full efficiency. He is also being offered more money at the accountancy firm, where he will head its Midlands VAT unit in Birmingham.
Coleclough will be replaced by Simmons' partner Heather Savage. She said: "People have different thresholds. What is perfectly acceptable to one person might not be for another."
Leading corporate tax partner James Savory cited similar reasons for his decision to leave Slaughter and May on 30 November to set up his own practice to "control my own hours".
Savory said: "You are in a culture where it is accepted that you work until seven or eight every night, you work sometimes at weekends and you are potentially on call at any time."