Allen & Overy is breaking with magic circle formality by offering staff the chance to dress down five days a week.
The move affects only Allen & Overy's London office, but head of corporate Richard Cranfield believes that it will spread to the firm's international offices.
Brief guidelines on the "smart casual policy" have been issued to staff, he says. "We are relying on people's good sense. People know what smart casual dress means."
The firm hopes the option will enable staff to be more in tune with clients in the professional services and financial sectors and increase comfort.
Allen & Overy joins a growing number of UK businesses adopting the US trend for a more relaxed style.
Linklaters & Alliance's staff can dress down for July and August and five more of the City's top 20 firms now encourage staff to dress down once a week, including Clifford Chance and Simmons & Simmons.
A number of A&O's investment bank clients, such as Goldman Sachs, have already adopted a dress down policy. Other major professional services firms have also opted for the casual look, such as Arthur Andersen.
Mike Orton, HR director at Morgan Cole, says: "IT firms are already very used to it. To some extent we are reflecting that. They are more comfortable when we dress down.
"We won a significant piece of business from an IT company after a pitch which involved dressing down. We believe that was instrumental. The other firms there were in suits."
But HR managers are keen to emphasise that a dress down policy does have its limits.
"If people wear scruffy jeans or a cut-off top we would have to have a word with them. I've had to do that once or twice, but on the whole people are very good," says Sue Cross, HR manager at Ashurst Morris Crisp.