In what were the first public comments from a senior U.S. official about America's free trade deal with the Great Britain, Michael Froman, U.S. Trade Representative said on Wednesday, 28 October that US is not keen on pursuing a separate deal with UK if the latter leaves the European Union (EU).

British voters are due to decide before the end of 2017 whether their country should remain in the EU or not. However, opinion polls have already shown rising support for leaving the bloc.

Froman's statements undermine a key economic argument deployed by proponents of a British exit that say the United Kingdom would prosper on its own and be able to secure bilateral free trade agreements (FTAs) with trading partners.

The United States is Britain's biggest export market after the EU, having bought over $54 billion in goods from the United Kingdom in 2014.

"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 in an interview, adding that EU membership gives Britain more leverage in negotiations.

"We're not particularly in the market for FTAs with individual countries. We're building platforms ... that other countries can join over time," he further said.

Froman added if Britain leaves the EU, it would face the same tariffs and trade barriers as other countries outside the U.S. free trade network.

"We have no FTA with the U.K. so they would be subject to the same tariffs – and other trade-related measures - as China, or Brazil or India," he said.

Washington has just sealed a major trade deal with 11 other Pacific nations and wants to wrap up negotiations with the EU on the Transatlantic Trade and Investment Partnership (TTIP) by the end of next year.

Other than goods, the United States is also Britain's second-largest export market for vehicles outside the EU.

If Britain is not part of the EU and therefore not part of TTIP, British cars exported to the United States, such as those made by Jaguar Land Rover, would face a 2.5% tariff and could be at a disadvantage to German and Italian-made competitors.

British exports of fuel and chocolate could also be at a disadvantage if TTIP abolishes tariffs on those products.