Nigel Farage lobbied "high-level" members of the White House on the Chagos deal ahead of Andy Burnham’s first meeting with Donald Trump, a Reform UK insider has said.
Mr Farage, who offered to provide Mr Burnham his advice before he jetted off to New York, had conversations with Washington figures over the past week.
The Reform UK source revealed Mr Farage was hoping the issue would be discussed in Mr Burnham’s meeting with Mr Trump.
They told The Telegraph that the US President’s rejection of the deal had proved Mr Farage’s work had "paid off".
In a blistering assessment of the "terrible" Chagos handover deal, Mr Trump told reporters last night: "Most of them say they have ownership.
"Somebody has ownership that never showed up before after decades and centuries. I think it’s ridiculous."
He added: "It’s strategically very important. Very important.
"We had to send our B2 bombers when we bombed the Iran nuclear [sites], we had to send the B2 bombers back home to Missouri, as opposed to flying for a short period of time.
"They had to fly for a long period of time. It’s amazing how strategic it is, and I think the Prime Minister is going to be looking at it."
After Mr Trump spoke out against the deal, the Prime Minister confirmed the handover agreement was under review.
Mr Farage had urged Mr Burnham to go into his first meeting with Mr Trump by taking a tough stance on the Chagos Islands.
"My advice would be simple", the Reform UK leader told GB News on Monday.
"Start off by saying that we’re not going to give away the Chagos Islands to a Chinese influence, Mauritius.
"Get the meeting off to a good start, and then you can start to talk tough about the Falklands and other things."
He added: "If Andy Burnham walks in there and says ’we’re going to continue giving China massive influence over this’, well, there are two things really.
"One is the Diego Garcia base, the second, of course, is the Chagossian people. But I think frankly, he needs to say that’s not going to go ahead."

World Affairs Correspondent