The Conservative chief whip has declined to remove a tarantula called Cronus from his office despite a House of Commons ban on pets.

Gavin Williamson, appointed by Theresa May as her parliamentary enforcer in July, has spoken in recent days about his unusual deskmate, who is kept in a glass tank and named after a Greek god who castrated his father and ate his children.

However, the publicity means the pet has come to the attention of the serjeant at arms, since Palace of Westminster rules state that the only animals allowed on the estate are guide and security dogs.

A source close to Williamson told the Sun: “The Commons authorities were told in no uncertain terms that Cronus was staying, as he is government business and this is not a Commons matter.

“The point was also made that when they remove all the mice here, we may then think about removing Cronus.”

Williamson spoke earlier this week about how the tarantula was part of his team. He said: “You have to look at different ways to persuade people to vote with the government and it’s great to have Cronus as part of the team. Everyone is obsessed with Palmerston and Larry the cat, but in the whips’ office we have a proper pet.

“I’ve had Cronus since he was a spiderling, so I have a very paternal sort of approach. It’s very much the same sort of love and care that I give to my spider as I give to all MPs.

“Cronus is a perfect example of an incredibly clean, ruthless killer – absolutely fascinating to rear.”