“Happy Brexit Day” notices claiming not to tolerate languages other than English are “racially aggravating”, according to police.
A resident of Winchester Tower in Norwich first spotted them at 06:00 GMT on Friday, as first reported in the Huffington Post.
“The the Queens [sic] English is the spoken tongue here,” the signs read, adding: “best evolve or leave”.
The typed pages – posted on all 15 floors – have since been taken down.
Norfolk Police said the matter was being treated as a “racially aggravated public order incident.”
A spokeswoman said posters kept by residents “have since been seized for forensic inquiries” and CCTV footage will be examined.
The UK officially left the European Union on Friday at 23:00 GMT after 47 years of membership, and more than three years after it voted to do so in a referendum.
The signs say: “We do not tolerate people speaking other languages than English in the flats.
“If you do want to speak whatever is the mother tongue of the country you came from then we suggest you return to that place and return your flat to the council so they can let British people live here.
It added: “We can return to what was normality before you infected this once great island.”
The Norwich City Council block of 95 flats houses tenants who are over the age of 55.
The resident who discovered a sign on Friday – and did not want to be named – said he alerted the caretaker, who removed them from the fire doors on every floor.
He said: “It’s heartbreaking not only for those it’s directed at but also for the person posting it.”
A police spokeswoman told the BBC: “There is no place in society for hatred and intolerance.
“Nobody should have to face intimidation because of who they are and it is more important than ever that we stand together in the face of hostility.”
The Labour MP for Norwich South, Clive Lewis, said the city will always “face down racism and bigotry”.
“Norwich is an overwhelmingly welcoming city to people from other countries,” he said.
“There was an element of the Brexit campaign which did utilise the concept of anti-immigration feeling and sentiment, and that manifests itself in messages like that.”