Four fire engines and around 25 firefighters were called to a fire at shops with offices above on Paternoster Square in St Pauls.
A small part of an electrical intake room on the ground floor was damaged by the fire. Around 400 people were evacuated from the site as a precaution. There were no reports of any injuries.
The Brigade was called at 1301 and the fire was under control by 1445. Fire crews from Dowgate, Whitechapel, Lambeth and Soho fire stations attended the scene.
The cause of the fire is believed to be accidental.