Front-runner for the White House Hillary Clinton has become the latest heavyweight American politician to wade into the referendum campaign and warn against a Brexit vote.
Clinton - who is almost certain to clinch the Democratic Party's presidential nomination, and leads in most polls for the November election to succeed Barack Obama - has made it clear she thinks it would be a mistake for the UK to go it alone.
The former secretary of state's senior policy adviser, Jake Sullivan, told The Observer: "Hillary Clinton believes that transatlantic cooperation is essential, and that cooperation is strongest when Europe is united. She has always valued a strong United Kingdom in a strong EU. And she values a strong British voice in the EU."
The latest foreign intervention came as Leave campaigners launched a furious backlash against Obama, branding him an irrelevant "lame duck" after the US president warned Britain would be at "the back of the queue" for American trade deals if it backed Brexit.
Justice Minister Dominic Raab led the personalised attacks against Obama as the Leave campaign tried to shift the emphasis of the campaign to immigration.
He said: "This is really about a lame duck US president about to move off the stage doing an old British friend a favour.
"I have got no doubt that future US trade negotiators are going to look to other opportunities - I think the British will be first in the queue, not at the back of the queue."
The attack came after Obama set out in stark terms that Britain would not be able to strike a free trade deal with the US "any time soon" if it leaves the European Union (EU) because Washington's focus would be on reaching agreement with Brussels.