The London Zoo gorilla that escaped from his enclosure reportedly walked out of an unlocked door and drank five litres of undiluted blackcurrant squash during his attempt to escape.

Silverback Gorilla Kumbuka remained outside of his enclosure for three hours on 13 October 2016 while the zoo went into lockdown. Some visitors were locked in the gift shop as authorities attempted to get the animal back into secure surroundings before subduing the animal with a tranquilliser dart.

Professor David Field, the Zoological Society of London (ZSL)'s zoological director, wrote a blog post explaining the doors to the enclosure had not been locked, so the animal had been able to roam out into a corridor, where he encountered a surprised zookeeper.

"We've since established that Kumbuka made an opportunistic escape from his unlocked den into the staff-only service corridor where a zookeeper was working," Professor Field said.

"Thanks to the incredibly close bond and relationship shared by the zookeeper and Kumbuka, the zookeeper was able to continually reassure Kumbuka, talking to him calmly and in the same light-hearted tone he would always use, as he removed himself from the area.

"Staff raised the alarm that triggered our standard escape response, while Kumbuka briefly explored the zookeeper area next door to his den, where he opened and drank five litres of undiluted blackcurrant squash.

"Kumbuka was immediately contained in the non-public area by quick-thinking zookeepers responding to the alarm, where he was tranquillised and moved back into his den."

The professor also added there had been no doors or windows smashed in the "opportunistic" escape, as some visitors to the zoo had feared.

At the time, eyewitness Brad Evans had told BBC Radio London: "We were having a cup of coffee in the main restaurant area when they locked us all in and said there was an incident. They gave us free teas and coffees and obviously we were asking what was going on and they told us that a gorilla had got out of its enclosure. As we were waiting we saw the police turning up in numbers with loads of guns."

The zoo is now establishing how and why the doors were left unlocked, reassuring visitors it took security seriously.

Professor Field added: "Any breach of our security protocols is incredibly serious, but the incident itself was less dramatic than some would have you believe. Within two hours Kumbuka was back with his family, snacking on treats, and probably wondering what all the fuss was about.

"The professional responses of our zookeepers, our security teams and the emergency services was exemplary and thanks to the zookeepers' strong relationship with Kumbuka they were able to reassure him during the incident."