Ted Cruz has attacked President Obama's intervention in the Brexit debate as a "slap in the face" to the United Kingdom.
Cruz, who is struggling to prevent Donald Trump's march towards the Republican nomination for president, said the UK would be at the "front of the line for a free trade deal" in the event it left the EU.
However the Texas Senator's comments were dismissed by Labour MP Mike Gapes, the former chairman of the Commons foreign affairs committee.
He told The Huffington Post: "Following the support for Brexit by Vladimir Putin and Marine Le Pen they now have this endorsement by a gun-toting right-wing ideologue who will not be US President. They are welcome to it. And I don't think it will help the quitters cause."
The pro-Brexit campaign was furious with President Obama after he used his visit to London last week to warn the UK would be at the "back of the queue" for any trade deal post-Brexit.
Writing for The Times, Cruz said Obama "has appeased America’s enemies, and has taken every opportunity to curry favour with those who seek to threaten our interests. The reality is that Obama reserves his harshest language for our friends while offering generosity and platitudes to our enemies".
"Instead of standing with our allies President Obama routinely hurls insults at them. Sadly, it happened in London last Friday, when the President of the United States informed the British people they would be at the 'back of the queue' for a US-UK free trade deal if they dared to vote to leave the EU on June 23.
"This was nothing less than a slap in the face of British self-determination as the president, typically, elevated an international organisation over the rights of a sovereign people."
James McGrory, the chief campaign spokesman for the Britain Stronger in Europe campaign, said Cruz's endorsement was not useful to the 'Leave' camp.
Trump beat out his rivals in Tuesday’s string of primaries, bringing the former reality television star closer to notching the magic number of 1,237 delegates needed to claim the GOP nomination and forestall a second round of balloting at the Republican National Convention in Cleveland.
Cruz and Ohio Gov. John Kasich, who began this week by announcing a coordinated effort to beat Trump, now look ahead to the May 3 Indiana primary in their effort to break his stride.