US trade representative Michael Froman has dealt the biggest blow to date to those campaigning for the UK to leave the European Union.
In the first public comments from a senior US official on future trade relations between the US and the UK, Froman said, "I think it's absolutely clear Britain has a greater voice at the trade table being part of the EU, being part of a larger economic entity."
He continued, "We're not particularly in the market for FTAs with individual countries. We're building platforms ... that other countries can join over time.
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"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."
Pro-European MEPs have seized on the comments, offering them as further proof the UK must remain in the EU.
Neena Gill, an S&D member of Parliament's EU-US delegation, said, "US Trade Representative Froman is absolutely right in pointing out that leaving the EU would be detrimental for UK interests in terms of trade - as would be the case for many other areas of major importance to the wellbeing of British citizens."
She continues, "By stating, in black and white, that the US is not in the market for FTA's with individual countries, Froman clearly debunks Eurosceptic claims that the UK would be better off negotiating its own trade agreements rather than instead of within an EU framework."
Referring to her own region, she offered a specific example of the impact Brexit would have on the local and economy and jobs.
She said, "The impact on my region of Brexit would be devastating. In that case, as Froman says, British cars exported to the United States would face a 2.5 per cent tariff and could be at a disadvantage to German and Italian-made competitors."
Froman's remarks are the second setback the 'OUT' campaign – spearheaded by the Vote Leave and Leave.EU groups – has suffered this week, following UK Prime Minister David Cameron’s open defence of EU membership on Wednesday.
Speaking to the UK Parliament, Cameron made a convincing case in rejecting the oft-used Eurosceptic argument that Britain could, like Norway, maintain close ties with the EU while remaining outside of the political bloc.
Cameron said, "Some people arguing for Britain to leave the European Union ... have particularly pointed to the position of Norway, saying that is a good outcome."
He continued, "Norway actually pays as much per head to the EU as we do, they actually take twice as many per head migrants as we do in this country, but of course they have no seat at the table, no ability to negotiate."
Cameron is currently attempting to renegotiate Britain's relationship with the EU. He is expected to target four key areas in the negotiations.
These are likely to be: British exemption from a commitment to an "ever closer union"; more sovereignty for national parliaments including a new "red card" system that allows UK to block unwanted EU law; increased protection for non-Eurozone countries; a clear statement confirming the euro is not the official currency of the EU.
Cameron's success - or otherwise - in achieving these demands is likely to play a key role in determining whether Britain remains in the bloc.