The fire broke out shortly after 0600 GMT at the Animal Adventure section before spreading to an adjacent gift shop and cafe, near an area where visitors can handle and feed animals, the zoo and London Fire Brigade said.
Staff on duty moved animals to safety and the fire was brought under control about three hours later.
"A number of zoo staff have been treated at the scene for smoke inhalation and shock," the zoo said in a statement. "Our staff are now in the process of assessing the situation in difficult conditions. At present one aardvark is currently unaccounted for."
The zoo, located in Regent's Park in central London, said it would be remain closed until further notice.
The attraction, the world's oldest scientific zoo which dates it origins back to 1826, houses 20,166 animals, according to its inventory for 2017.
"The fire mainly affected the cafe and shop," said Station Manager Clive Robinson. "Part of nearby animal petting area was also affected although at this stage we don't know if any animals were involved."
(Reporting by Michael Holden; Editing by Edmund Blair and Stephen Powell)
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)
Get the latest election news, live updates and election schedule for Lok Sabha Elections 2019 on ndtv.com/elections. Like us on Facebook or follow us on Twitter and Instagram for updates from each of the 543 parliamentary seats for the 2019 Indian general elections.