The US has stressed the importance of the UK's membership of the European Union, days after David Cameron signalled he would be prepared to lead the country out of the 28-member bloc unless there are major reforms.
Washington's ambassador to the UK Matthew Barzun said Britain's membership of the EU was a "really good thing" from an American point of view, and the prospect of a transatlantic trade deal offered a "big prize" to the US and Europe.
He said US leaders "really value a strong United Kingdom in a strong EU" because the nations see "eye to eye" on a range of crucial international issues.
The Prime Minister has vowed to renegotiate the UK's relationship with Europe ahead of an in/out referendum by the end of 2017 if he is returned to Downing Street after the general election.
In a landmark speech less than a fortnight ago setting out plans to tighten welfare rules for EU migrants, Mr Cameron warned that he will "rule nothing out" if other countries turn a deaf ear to British concerns.
At a Westminster lunch, Mr Barzun was asked whether a British exit would reduce the importance of the UK's special relationship with the US.
Mr Barzun said: "If you ask us, we really value a strong United Kingdom in a strong EU because we see eye to eye ... on the important issues of the challenges we face - look at Isil, Ebola, climate change, Russian aggression - Europe and the UK are the first places we turn to deal with that.
"That's on the negative side. On the positive side, look at the transatlantic trade deal.
"We see eye to eye. So from a selfish point of view it's a really good thing."
But he said it was "entirely up to you folks" to decide on the relationship "with and within Europe".
He also dismissed "fear-mongering" about the potential impact of the transatlantic trade and investment partnership (TTIP), which critics have warned could open up the NHS to US companies.
Mr Barzun said: "N othing in this is intended to lessen the state's power to provide services for its people.
"So the fear-mongering around what would happen to the NHS is just not true, it's not part of TTIP, full stop."
A transatlantic deal could help drive up environmental standards, labour rights and safety around the world, he added.
"What I think we ought to be doing is pulling back and saying, 'think for a moment the high environmental, labour and consumer safety - just to name three big ones - standards that the UK has, that Europe has and the United States all have'.
"I know they are not the same, but my goodness, pull back and look at it in the context of the whole global system. Can't we figure out a way to do a deal that keeps these standards high, sets some even higher, so that we support and grow jobs for our economies but also set a standard for the rest of the world to plug in.
"I think we need to look at it in that bigger picture. I'm not minimising it, I know the details of the disagreements are real, but we need to work through them in the context of the big prize that exists if we can do it."
Mr Barzun defended US technology giants over allegations that they minimised the tax they paid and failed to do enough to prevent terrorists from communicating on their networks.
Asked whether Facebook, Amazon and Google were tax dodgers, the ambassador said some firms were "clever about using the rules that exist" but it was governments in the US and UK that had written those rules.
"We made these rules and they are playing by them," he said.
"My hope would be that if and when rules change they will play by those new rules as well."
The family of murdered soldier Lee Rigby have said they hold Facebook partly responsible after a report by the Intelligence and Security Committee found it failed to take action over an online chat in which one of the extremists vowed to kill a soldier.
Mr Barzun said: "Are you right to blame them? No."
On the role that internet firms play in tackling wider online crime he added: "I don't think it's fair to pin everything on the private sector, nor is it fair to say they have no responsibility in trying to come up with solutions, because of course they do - and they want to and they need to."
Mr Barzun spoke of the enduring importance of the links between the UK and US, saying "this special relationship is vibrant, it is versatile and it is vital".