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Palmerston, the Foreign Office’s much-loved mascot and chief mouser, is returning to Whitehall after a four-month purr-dah brought on by stress.

The black-and-white rescue cat arrived at the FCO’s King Charles Street base in April 2016 and swiftly became an online celebrity with nearly 100,000 Twitter followers of @DiploMog.

But he disappeared from frontline duties during the summer with the reasons for his sudden departure a mystery until now.

A diet too rich in Dreamies treats, a territory too large, and being picked up and woken too often were identified as factors in his decline.

Special rules, dubbed the “Palmerston Protocols”, are now being introduced to ensure he can once more stalk the hallowed corridors in peace.

“As much as the Foreign Office loved Palmerston, it was no longer an environment that was working for him,” the FCO’s top mandarin Sir Simon McDonald explained to staff this morning. “He was over-grooming on his front legs, a sign of stress.”

So Palmerston, named after Britain’s longest-serving Foreign Secretary, was moved in July on vet’s advice to live in a more traditional environment, the home of one of Sir Simon’s assistant private secretaries.

“During his summer holiday, Cabinet ministers, colleagues and overseas visitors have asked me anxiously about his whereabouts,” continued Sir Simon in a blog. “The good news is that Palmerston is coming back this week. But we must remember why he needed a break, and change our behaviour towards him.

“He is happy, healthy and full of energy. His pelt is glossy and mostly grown back. We need now to keep him that way.”

Here are the Palmerston Protocols being introduced to keep the cat happy and healthy:

Clause 1: No-one (apart from his carers) should feed Palmerston.

No Dreamies. No bowls of food under the desk for when he happens to drop by. Nothing!

Clause 2: Everyone must help keep Palmerston in the “Palmerston Zone”.

“Cats are territorial. They fret when their territory is bigger than they can manage,” Sir Simon told staff.

“They can cope with an ever smaller territory as they age. Palmerston has been king of King Charles Street, roaming from basement to fourth floor (with quad, Downing Street and occasionally St James’s Park thrown in) for nearly four years. We think he’s about six years old, ie entering feline middle age.”

On veterinary advice, a more “manageable territory” has been marked out - the offices and area surrounding the Grand Staircase. Heavy doors mark the limits with signs saying: “You are entering/leaving the Palmerston Zone”.

On veterinary advice, a more “manageable territory” has been marked out — the offices and area surrounding the Grand Staircase.

Clause 3: Everyone must respect Palmerston’s personal space.

Sir Simon stressed: “Don’t wake him if he is sleeping. He has full choice of who he deigns to greet or imperiously ignores.”

Sir Simon stressed: “Allow Palmerston to choose whether he wants to interact with you: offer your hand as if you were introducing yourself to a stranger, and allow Palmerston to make the first move.

“Don’t wake him if he is sleeping. He has full choice and control of who he deigns to greet or imperiously ignores.”

Clause 4: “My staff office will serve as both my Outer Office and Palmerston’s refuge: Palmerston HQ,” said Sir Simon.

“If he is in Palmerston HQ, he is not to be disturbed. Palmerston is a friendly, outgoing cat, but we all need our privacy. Like Greta Garbo, sometimes he wants to be alone.”

“If he is in Palmerston HQ, he is not to be disturbed. Palmerston is a friendly, outgoing cat, but we all need our privacy. Like Greta Garbo, sometimes he wants to be alone.”

Palmerston arrived at the FCO from Battersea Dogs and Cats Home in April 2016 and swiftly became a celebrity with nearly 100,000 Twitter followers of @DiploMog, including over 11,000 in Japan and his photo is on cat food in Australia.

“It is a testament to his popularity that, for most people these days, Palmerston is our black-and-white cat rather than our longest-serving Foreign Secretary,” explained Sir Simon.