The black-and-white rescue cat said that after four years of service it is time for him to “step away from the limelight” and enjoy some “me-time” in the country.
The playful moggie has become an online sensation since he arrived at the FCO’s King Charles Street base in 2016, amassing more than 105,000 Twitter followers.
But, on Thursday, an open letter was shared on the platform, saying it was time for him to take a well-earned paws from duties.
He wrote: “The spread of coronavirus around the world has caused many, like me, to begin working from home. I assure you I have been as diligent as ever.
“While I have not been able to catch the King Charles Street mice from afar, my diplomatic efforts on engaging the mouse species have seen a significant uptick.”
The announcement continues: “I have found life away from the front line more relaxed, quieter, and easier.
“I have enjoyed climbing trees and patrolling the fields around my new home in the countryside.”
He stressed: “Of course I love the hustle and bustle of the office. I will miss hearing the footsteps of an Ambassador and sprinting to my hideout to see who it is.”
But, he said: “As I grow older, I must take a step back from diplomatic duties and enjoy some me-time.”
He ended his note by thanking his followers and pledging to remain an ambassador for the UK and the Foreign Office.
“I hope that I have done you proud in putting the UK’s best foot, or paw, forward in each interaction,” he added.
“My 105,000 Twitter followers show that even those with four legs and fur have an important part to play in the UK’s global effort.”
The FCO’s permanent undersecretary Sir Simon McDonald paid tribute to Palmerston, as scores of fans wished the cat well on his new chapter.
Sharing a photo of himself sat beaming alongside his colleague, Sir Simon tweeted: “In 2016 Palmerston arrived from Battersea, mouser & social media phenomenon, with 105K Twitter followers.
“After four-and-a-half happy years Palmerston retires at end of August: he’s enjoyed lockdown life in countryside so much, he’s decided to stay.
“Everyone at the Foreign Office will miss him.”
Fellow FCO Office shared their own farewell messages, with Jon Benjamin, director of the department’s Diplomatic Academy, wishing him a “very happy retirement”.
“He left us a slightly chewed dead mouse next to my desk in the UK’s Diplomatic Academy once, and we were of course not very grateful,” he added.
Caron Rohsler, British high commissioner to the Maldives, tweeted a goodbye message from her embassy’s own diplomatic feline, Miska.
It read: “Congratulations on your exemplary service to British foreign affairs, and for fur-thering the cause of diversity in our noble institution.
“I’m sure an elevation to the pawrage cannot be far off.”