Palmerston was rumoured to be unwell and close to death at one point in October.
It has now emerged that the cat was showing signs of stress after civil servants carried on picking him up and feeding him too many treats.
In a series of tweets, Palmerston announced his return and some new protocol.
His official account wrote: “I am happy to announce that I will be returning to my Chief Mouser duties at the @foreignoffice this week! New guidance – the Palmerston Protocols – will govern my care in the FCO to make sure it’s working for me.
“The Palmerston Protocols are: (1) no-one other than my carers should feed me – especially no treats! (2) everyone should respect my personal space and allow me to lead interactions. (3) Palmerston HQ will house my bed, toys and food. I like privacy in Palmy HQ.
“Everyone should help to maintain the Palmerston Zone. The FCO is a very big, old building and I was finding it difficult to maintain a territory so large. My new Zone is much more manageable, and has lots of comfy seats for naps.
“The Palmerston Protocols are designed to ensure my welfare and happiness in the FCO. My care is under constant review by a dedicated team of voluntary carers, who will be monitoring whether the Palmerston Protocols work. Mice – watch out! I’m coming back!”
According to The Daily Telegraph, Palmerston was taken out of the Foreign Office in June by Sir Simon McDonald’s private secretary after becoming overweight and licking all the fur off his front legs.
The overgrooming can be a sign of stress in cats, and it is a habit which can be hard for them to stop.
An email to civil servants said his diet was now regulated and his coat has mostly grown back and is “glossy”.