The comments from US trade representative Michael Froman come amid a rise in Euroskeptic support in Britain, and mark the first time a senior American official has commented on the British referendum on EU membership, to be held before the end of 2017.
Big blast of reality for Brexit case from US trade chief Michael Froman…no US interest in trade deal with UK alone https://t.co/punJ8xq6aX— John Healey MP (@JohnHealey_MP) October 29, 2015
Froman dismissed the claims of many pushing for a Brexit, who have argued that without the restraints of EU membership, the UK would be better placed to negotiate free trade agreements with countries that better suit Britain's strategic interests.
"I think it's absolutely clear that Britain has a greater voice at the trade table being part of the EU, being part of a larger economic entity," Froman told Reuters, arguing that he believed London would have greater negotiating clout if it remained in the bloc.
"We're not particularly in the market for FTAs with individual countries. We're building platforms… that other countries can join over time."
The US is Britain's second largest export market after the EU, with the US buying more than US$54 billion (£35 billion) worth of UK-goods in 2014 alone.
Adding to the concerns of those fearful of a Brexit, Froman said that if Britain was to leave the EU, the country would be exposed to the same tariffs and trade barriers as other countries without US trade deals.
"We have no FTA with the UK so they would be subject to the same tariffs — and other trade-related measures — as China, or Brazil or India."
Froman's intervention marks the second high profile warning about the dangers of a possible Brexit in as many days, after UK Prime Minister David Cameron warned against those pushing for the UK to pursue a Norway-style association agreement with the EU.
While many Euroskeptics have called for Britain to try and adopt a similar model to non-EU countries like Norway and Switzerland, Cameron said such a move would have a number of economic drawbacks.
"Norway has to pay hundreds of millions of euros in membership fee to access the single market," he warned.
Those campaigning for a Brexit have labeled Cameron's and US trade representative Froman's recent interventions as merely part of a scare campaign to try and keep Britain in the EU amid rising Euroskepticim.
Former Conservative-turned UKIP MP Douglas Carswell said Cameron's change in tone over the Brexit debate was an indication that the PM wasn't confident of securing a new membership agreement with the bloc.
He told the Express that Cameron's intervention was "almost an admission that he's not going to get new terms, because he's trying to persuade us on the current terms.
"He's trying to make the case on the basis of how things are now rather than pretending he's going to get some super duper new deal.
"When David Cameron first announced an In/Out referendum he was talking about fundamental change. Now he's reduced to talking about roaming charges and cheap flights.
"He [Cameron] is having to justify the whole membership of the EU in these terms because he knows he's not going to get any significant new deal."
While polls suggest that the majority of Britons would prefer to remain tied to Brussels, there has been a spike in anti-EU sentiment and support in recent times, following the launch of various Brexit campaigns and the EU's handling of the Greek debt and refugee crises.