Andy Burnham sent messages to a fake White House adviser after being sent the number from David Lammy, it has been revealed.
The Prime Minister was in contact with an unknown individual posing as top White House aide Susie Wiles before concerns were raised about the incident.
Downing Street denied that Burnham initiated contact.
Instead, it is believed that former foreign secretary David Lammy fell for a similar hoax, and handed the PM the number after being contacted by someone claiming to be Wiles asking if he could pass on their details to the prime minister.
Lammy reportedly obliged, and the contact exchanged messages with Burnham. Shortly after first contact was made, Burnham notified official government channels.
No significant messages were sent, and the suspicious activity was quickly reported to the appropriate authorities, it is understood.
This is not the first time someone has tried to impersonate Wiles, who is understood to have changed her number for security purposes this year.
The British embassy in Washington was so concerned it raised the matter with the White House, two officials told Politico, which first reported the exchange.
Last year, The Wall Street Journal reported that the FBI had launched an investigation into efforts to access Ms Wiles’ phone after an unknown person contacted prominent Republicans and business chiefs while pretending to be her.
It is unclear when the messages were sent and what was discussed between Mr Burnham, who succeeded Sir Keir Starmer in Downing Street on July 20, and the impersonator.
A White House official said: “White House staff are routinely trained on best practices to protect the integrity of their devices, and the entire administration has robust security protocols.”
A UK Government spokesperson said: “We do not comment on national security matters.”
Mr Burnham has not yet met with Mr Trump since becoming Prime Minister, but the US President said he thinks they are “going to have a very good relationship”.
The US president said the UK was “sitting on top of gold”, and that in a phone call on his first day in office the Prime Minister had “sounded terrific” on allowing the fossil fuel to be tapped.
