The union and European flags outside the European commission building in London. Photograph: Neil Hall/Reuters
Voters are due to decide by the end of 2017 whether the UK should remain in the EU, and opinion polls show rising support for leaving the bloc.
The US is Britain’s biggest export market after the EU, buying more than $54bn (£35bn) in goods from the UK 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, 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.”
If Britain left the EU, Froman said, it would face the same tariffs and trade barriers as other countries outside the US free trade network.
“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,” he said.
Washington has just sealed a 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.
The US is 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 US, 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.