Home Secretary Amber Rudd was tricked into a brief email exchange with a prankster who impersonated the PM's new communications chief Robbie Gibb.
The hoaxer, who has previously fooled White House officials, managed to communicate with the minister before she spotted the wind-up.
According to The Guardian, he emailed her using her publicly-available contact details and she replied from her private email address.
The Home Office played down the trick.
"The home secretary does not use her personal email address to discuss government business," a Home Office source said.
"As the email exchange shows, she rapidly established that this was a hoax and had only exchanged pleasantries up to that point."
According to a transcript of the exchange, dated 7 July, Ms Rudd was asked which projects she was working on, and replied: "Well, as you can imagine a few things on the agenda but getting tough on people impersonating others is definitely up there."
Earlier this month, the prankster, who calls himself Sinon Reborn, reportedly fooled top White House officials into engaging in fake email exchanges.
The self-styled "email prankster" convinced a senior cyber security adviser he was President Donald Trump's son-in-law, Jared Kushner, according to CNN.
He also goaded the then media chief, Anthony Scaramucci, in the guise of ex-chief of staff Reince Priebus.
Eric Trump, too, was briefly hoodwinked by the prankster emailing as his older brother, Donald Trump Jr, about a long-range hunting rifle.
But Donald Jr soon realised it was a scam and replied: "I have sent this to law enforcement who will handle from here."