Comedian and author David Walliams had his post stolen "including a large box of letters from fans", he said on Twitter. The Britain's Got Talent judge apologised to fans that had written and not received a reply and urged them to write again.
The Little Britain star told his 1.65m followers that the mail had been sent via Harper Collins, the publisher which he has written six books for, three going on to become children's best sellers. He is set to release a new children's novel later this year.
Scotland Yard said the post was found by a member of the public in Camden, north London, adding that officers were "in the process of contacting the intended recipients and enquiries are underway to establish whether an offence has been committed".
There have been no reported cases of mail being stolen from celebrities in recent years. However, two years ago a trove of nude celebrity images were hacked from Apple's iCloud two years ago and posted online.
Dubbed the "Fappening", a lewd reference to masturbation, images of film stars Jennifer Lawrence, Kirsten Dunst and Kate Upton were released, along with those of singers Rhianna and Avril Lavigne and socialites Paris Hilton and Kim Kardashian.
"It is not a scandal. It is a sex crime," Lawrence told Vanity Fair shortly after their proliferation online. "It is a sexual violation. It's disgusting. The law needs to be changed, and we need to change… Just the fact that somebody can be sexually exploited and violated, and the first thought that crosses somebody's mind is to make a profit from it. It's so beyond me."
One of the hackers, Ryan Collins, pleaded guilty to one count of felony computer hacking and another for unauthorized access to a protected computer to obtain information for his role in the hack in March 2015, according to the Fox 43 TV station.
According to the plea document, the 36-year-old was responsible for hacking into more than 100 Apple iCloud and Gmail accounts, 18 of which belonged to celebrities, using a phishing scam.
Collins struck a plea deal, and could face up to five years in prison when sentenced later this year but prosecutors said they will recommend he only get 18 months in exchange for a guilty plea.