Britain is "in need of leadership" as it tries to navigate a painless exit from the EU, the US ambassador to London said on Monday.

Ambassador Woody Johnson also played down the possibility of London and Washington quickly striking a new trade agreement once Brexit enters into force on March 29.

The comments dealt a blow to Prime Minister Theresa May, who has argued that Britain will be able to strike new and more advantageous trade accords without the EU.

"I've been all over Wales, I've been all over Ireland and Scotland and also England -- and I am feeling that the country is in need of leadership," Johnson told BBC radio.

He added that some of the blame rested with parliament, which has balked at backing the draft withdrawal agreement May has struck with Brussels.

May was forced to abort a December debate about the arrangement after admitting it faced heavy defeat.

"You can see the frustration in the members of parliament in trying to navigate what the people wanted when they voted on the referendum," Johnson said.

May's government is ramping up preparations to avert chaos should Britain split from the EU without an agreement on future trade ties.

The UK leader also insists that her teams are busy negotiating new trade agreements with major trading partners such as the United States.

But Johnson, who has served as US President Donald Trump's envoy to the UK for a year, said London's draft Brexit arrangements with Brussels appeared to rule out the possibility of a separate deal with Washington.

"We are still going through the stages of deciding where exactly the country is going," he said.

Asked if a US-UK trade agreement was possible under existing draft Brexit terms, Johnson said: "It doesn't look like it would be possible."

Britain will remain closely tied to EU trade rules for a transition period running through the end of 2020 under the draft agreement, which parliament is set to vote on in January.

These preclude states from striking their own deals with countries outside the EU.

A Downing Street spokeswoman, asked to respond to Johnson's comments, said: "We want an ambitious trade agreement and we stand ready to conclude such an agreement as a priority after we leave the European Union."

In November, Trump warned that May's deal with the EU might hamper a US-UK trade deal if it went through.

"We have to take a look at seriously whether or not the UK is allowed to trade," Trump said at the time.

"As the deal stands, they may not be able to trade with the US and I don't think they want that at all." AFP