Trump to visit Britain amid Brexit turmoil

US President Donald Trump is due to fly into Britain for talks with the leader of the United States' closest ally in Europe and tea with Queen Elizabeth, against a backdrop of protests and what he has described as the UK's turmoil over Brexit.

Fresh from a NATO summit where Trump chided Germany and other European nations for failing to contribute enough to defence spending, British Prime Minister Theresa May is hoping his trip will boost the close ties between their two nations and help forge a future free trade deal.

"There is no stronger alliance than that of our special relationship with the US and there will be no alliance more important in the years ahead," May said in a statement, ahead of Trump's arrival on Thursday.

Trump's trip coincides with a tumultuous week for May after two senior ministers resigned in protest at her plans for trade with the European Union after Britain leaves next March.

The president has already waded into the debate saying Britain was "in somewhat turmoil" and that it was up to the people if she stayed in power.

He also said he might speak to Boris Johnson, who quit as foreign secretary over May's plans for a business-friendly Brexit which was only agreed by her cabinet last Friday after two years of wrangling.

Trump has long been a supporter of Brexit and has expressed enthusiasm for a wide-ranging trade deal with Britain after it leaves the EU, something heralded by Brexit supporters as being one of the great benefits of exiting the bloc.

"Our trade and investment relationship is unrivalled - we are the largest investors in each other's economies and every day a million British people go to work for US companies in the UK and a million Americans go to work for UK companies in the US," May said.

"This week we have an opportunity to deepen this unique trading relationship and begin discussions about how we will forge a strengthened, ambitious and future-proof trade partnership."

But despite May's warm welcome, a YouGov poll on Wednesday showed 77 per cent of Britons had an unfavourable opinion of Trump with just 50 per cent thinking his visit should go ahead.

A high metal fence has been erected around the US ambassador's central London residence where Trump will spend Thursday night and the embassy has sent out an alert warning Americans in London to keep a low profile in case protests turn violent.

More than 50,000 people have signed up to demonstrate in London on Friday when protesters intend to fly a large balloon over parliament portraying Trump as an orange, snarling baby.