The Downing Street cat will be spared eviction from Number 10 when the Cameron family leave, it has been confirmed.

Chief mouser Larry - a brown and white tabby entrusted with the rat-catching portfolio - will continue to reside in his famous abode when Theresa May moves in after becoming British Prime Minister on Thursday NZT.

A Cabinet Office spokeswoman said: "It's a civil servant's cat and does not belong to the Camerons - he will be staying".

Larry, who was re-homed from Battersea Dogs and Cats Home in 2011, was said to have a "strong predatory drive" that suggested he would be well-suited to the task of rat catching.

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David Cameron welcomed the cat's arrival, and said he would make a "great addition" to the Number 10 team.

Larry was the first cat to hold the appointment since stalwart ratter Humphrey was retired in 1997.

Humphrey was succeeded for a short period by Sybil, the pet of then Chancellor of the Exchequer Alistair Darling, in 2007 - but the renowned mouser failed to settle and returned to Scotland.

The Foreign Office appointed Palmerston, a moggy, in April to keep pests in Whitehall at bay.

An unofficial Twitter account set up under Larry's name posted today: "Does anyone know a good cat groomer available tomorrow? I need to make a good impression on someone ..."

It had earlier tweeted: "One final thing before they'll let you have the job @TheresaMay2016 - tell the people you'll keep me in Number 10."

- PA, AAP