Drunken guests at popular London Zoo events 'Zoo Lates' have allegedly been harassing and injuring the animals, leading to calls from animal rights groups for the evenings to end.

A joint letter to the director of the Zoological Society of London from animal rights charities, including the RSPCA, describes how one guest reportedly poured beer over a tiger, while a another tried to undress and enter the penguin enclosure.

In one alleged incident, a guest fell and “accidentally” punched a bird.

Detailing the “traumatising” events, the letter also describes how a male guest reportedly asked a staff member: “Which penguin can I fight?”

Yet another guest was accused of getting “touchy feely” with some baby penguins.

David Field, London Zoo’s zoological director, said; “While we appreciate organisations like this taking such an active interest in animal welfare, we can only reiterate that animal welfare is our top priority at all times.”

The letter, signed by the RSPCA, PETA, Animal Aid, the Born Free Foundation, OneKind, the Captive Animals' Protection Society, and Viva!, also says allowing visitors inside the zoo outside normal hours disrupts the animals sleep patterns causing them distress.

Mr Field denied the allegations, claiming: “We go to great lengths to ensure the normal routines and behaviour of our animals are not compromised by Zoo Lates events, and that visitor behaviour is monitored and managed carefully.”

More than 6,000 people have attended a Zoo Lates event, raising around £800,000 annually for London Zoo.

“Plying thousands of people with alcohol on what is actively promoted by the zoo as a 'wild night out' might be a good fundraiser for the London Zoo, but it doesn't put the animals' needs first", said PETA director Mimi Bekhechi.