The United States ambassador to the UK has called on Britain to side with President Donald Trump on Iran or risk "serious trade consequences" for UK businesses.

In a pointed intervention into an issue that has strained ties between the two allies, Woody Johnson said the UK should embrace Trump's hard-hitting sanctions on Iran, reimposed last week, and break with its European partners who are seeking to preserve the deal to curtail Iran's nuclear program.

“America is turning up the pressure and we want the UK by our side. It is time to move on from the flawed 2015 deal,” Johnson wrote in the UK's Sunday Telegraph, referring to the 2015 Obama-era pact agreed by the US, Iran, Britain, France, Germany, China and Russia. Trump withdrew from the deal in May.

“We are asking global Britain to use its considerable diplomatic power and influence and join us as we lead a concerted global effort towards a genuinely comprehensive agreement,” Johnson added.

In a joint statement last week, Britain, France and Germany said the Iran deal was “working and delivering on its goal” and said they “deeply regret” the reimposition of US sanctions. Europe has attempted to counteract the effects of the sanctions by launching an updated version of its “Blocking Statue,” a measure intended to protect EU companies doing business in Iran from being hit by punitive US measures.

On Sunday, Johnson seemingly disregarded the move, urging British businesses directly to cut ties with Iran. Johnson's comments come less than a week after the Trump administration on Tuesday reimposed a raft of sanctions on Iran that affect, among other things, the purchase or acquisition of US dollars by the Iranian government, the country's auto industry and trade in gold or precious metals.

Another phase of US sanctions will be reimposed in November and will target Iran's crucial oil industry.