Firefighters have been called to Jamie Oliver's Barbecoa restaurant in the City of London following an incident reported at 10.50am.
Four engines and 21 firefighters are battling the blaze at the restaurant, which describes itself on its website as "a celebration of traditional fire-based cooking".
There are as yet no reports of injuries and a spokesperson for the London Fire Brigade described the cause of the incident as unknown at this stage.
A call made to the restaurant's reservation line confirmed that Barbecoa is not taking bookings for today or tomorrow as a result of an "accident".
Jamie Oliver opened Barbecoa in the One New Change shopping centre in October 2010.
A spokesperson for Jamie Oliver said: "There was a very small fire that was quickly brought under control."