The presidential hopeful's support comes following the controversial intervention by Barack Obama
US presidential hopeful Hillary Clinton has joined the battle over Britain's EU future by throwing her support behind the campaign to remain in the European Union.
Her intervention came a day after President Barack Obama bluntly told the UK it should remain in the EU to preserve its remaining global clout.
Mrs Clinton's senior policy adviser Jake Sullivan told The Observer: "Hillary Clinton believes that transatlantic co-operation 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."
Mrs Clinton, who spent four years as US secretary of state during Mr Obama's first term, has made her foreign policy credentials a central piece of her campaign for president in the November 2016 race for the White House.
Responding to the comments, a 'Vote Leave' spokesman said: "Hillary isn't standing on a ticket of handing power over America's borders, economy and trade policy to a foreign power so her comments on the EU will sound hollow to anyone fed up of handing Brussels £350m a week."
Mr Obama, whose three-day visit to the UK has drawn to an end, infuriated the Leave camp at the beginning of his trip by telling Britain it would be at the "back of the queue" for any US trade deal if it voted to quit the bloc.
Mr Obama said that generally such economic agreements were difficult due to "parochial" interests and "factions" within countries.
The UK's justice minister Dominic Raab branded the President an irrelevant "lame duck" after his remarks.
"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."
UKIP leader Nigel Farage also savaged Mr Obama's comments, saying the President's intervention was at the "bidding of Cameron" and accused him of "doing his best to talk down to Britain".
London mayor Boris Johnson accused Mr Obama of "hypocrisy" and said his comments were "perverse".
"I think this is all a complete distraction. An attempt by the Remain campaign to throw dust in people's eyes," he said.
"Over the last few days, nobody on that side of the argument has been able to answer the key point that I have been making which is that it is inconsistent, perverse and yes, it is hypocritical of the United States to tell us that we should sacrifice more of our independence than they would ever dream of doing themselves."
After his controversial remarks, Mr Obama took on an altogether more positive tone and was warmly received at a town hall-style meeting with young people in London yesterday.
He praised the close relationship between the US and the UK, which he said had improved dramatically since the British "burned down my house" - a reference to the torching of the White House in the war of 1812-1814.
The US president insisted now was the best time in human history to be alive as he urged the audience to ignore cynical voices saying that nothing could change.
"Take a longer, more optimistic view of history," he said.
Mr Obama went on to have talks with the UK's Labour leader Jeremy Corbyn, who said afterwards that he had an "excellent" discussion with the President, with subject matter including inequality and the impact of technology and global corporations on world populations.
Later on Saturday, the President enjoyed a game of golf with British Prime Minister David Cameron before their pair enjoyed a private dinner hosted by US ambassador Matthew Barzun.
Mr Obama will be heading to Germany later for the last leg of his tour of the Middle East and Europe.