Andy Burnham exchanged messages with someone posing as one of Donald Trump’s top aides.
The prime minister spoke over message to who he believed was White House chief of staff Susie Wiles, before the suspicious activity was reported to authorities.
The British embassy in Washington was so concerned it raised the matter with the White House, according to Playbook, which first reported the exchange.
It is understood that Mr Burnham did not speak over the phone to the person impersonating the US president’s top aide. It is unclear what messages were exchanged, but it is understood none were of significance.
A government spokesperson said: “We do not comment on national security matters.”
Last year, the US government launched an investigation after elected officials, business executives and other prominent figures received messages from someone impersonating Ms Wiles.
“They breached the phone; they tried to impersonate her,” Trump told reporters at the time. “Nobody can impersonate her. There’s only one Susie.”
A White House official confirmed the investigation Friday and said the White House takes cybersecurity of its staff seriously.
The Wall Street Journal reported that senators, governors, business leaders and others began receiving text messages and phone calls from someone who seemed to have gained access to the contacts in Wiles’ personal cellphone. The messages and calls were not coming from Wiles’ number, the newspaper reported.
Mr Burnham secured an early endorsement from Mr Trump, with the discussion taking place within an hour of Mr Burnham’s arrival at Downing Street last month.
It comes after relations between the US president and Mr Burnham’s predecessor, Sir Keir Starmer, grew cold in the wake of Mr Trump’s war on Iran and disagreements over drilling in the North Sea.
Speaking in the White House days after Mr Burnham entered No 10, the US president said Mr Burnham told him that he was going to “open up North Sea oil”.
Speaking in the White House, the US president said if the prime minister followed through on his commitment the UK was “going to be a wealthy country”.
The US president, a vocal supporter of fossil fuels, has long championed the move as he has railed against “windmills”.
Mr Trump had lost a legal battle with the Scottish Government to stop a wind turbine development visible from his Aberdeenshire golf course.
The president also argued Mr Burnham would have to tackle the issue of immigration.
Mr Trump said: “He’s going to have to mention it because it’s killing your country.
“I know Starmer did very little about it, and they’re coming from all over the world.