US Trade Representative Jamieson Greer said the British government's adherence to EU rules posed "a problem" for the country's goal of expanding a trade deal struck with President Donald Trump's administration last year.
His comments come as British Prime Minister Andy Burnham looks to build on his predecessor Keir Starmer's efforts to forge closer post-Brexit cooperation with the European Union.
Asked whether the UK was using the "freedom" afforded by its departure from the EU at the start of the decade, Greer told the Financial Times: "No, they're not."
"Based on my conversations with the UK, where I say, 'Hey, you're free of the EU. Why don't you liberalise a little bit, accept goods made to American standards, accept agricultural goods made to American standards?'" he said.
"They say, 'Well, we have to wait and see what the EU says about it.' You know that is a problem for us, and that's challenging."
During a visit to London this week, French President Emmanuel Macron said Paris and London "remain open to discussing everything, including advancing the UK reset with the European Union".
A crucial summit between the EU and the UK is expected before the end of the year.
Burnham told journalists on Wednesday that since Brexit, "people here are fed up with the [lack of] vision, people see that we need to work closer together".
Burnham also told the British parliament this week, in his first address as prime minister, that Brexit had contributed to sluggish UK economic growth.
Trump and Starmer announced a trade deal in May 2025 that saw Washington lower tariffs on British luxury cars and on steel and aluminium after the US president unleashed a barrage of tariffs on countries around the world.
The UK government insisted that the deal to allow in more US agricultural products, including ethanol and US beef, would not dilute British food standards, amid concerns over chlorinated US chicken and hormones in its beef.
The UK has also reached a deal to avoid US tariffs on medicines by agreeing to pay higher prices for certain drugs.
