The mayor of London, Boris Johnson, has announced a rise to the living wage in London to £9.15 an hour.
The voluntary rate is £1.30 higher than the rate for the UK as a whole, reflecting the higher cost of living in the capital.
Some are calling for the wage to be mandatory, claiming that nine out of ten big businesses in London do not pay it.
Boris Johnson said the number of companies paying the voluntary rate had doubled over 12 months to exceed 400.
Mr Johnson said: "In excess of 400 businesses have made the commitment [in London], but we need even more converts, particularly in the retail and hospitality sectors."
By law all firms must pay the national minimum wage of £6.50 an hour, but they are urged to give employees the living wage, set at a level supposed to allow a more comfortable life.
Rhys Moore, director of the Living Wage Foundation, says employers who already pay the living wage are seeing the benefits.
"Their experience is that if you invest in your people they tend to be happier, they work harder, they're more productive and they will stay with the business," he says.
"You don't have to rehire, retain, and spend money on those costs. It's good for them and good for the bottom line."
