US Secretary of State Mike Pompeo arrived in London on Wednesday seeking reassurances over China that could let the two old allies continue sharing intelligence and strike a post-Brexit trade deal.

The first leg of a five-nation tour that also takes in Ukraine comes with Britain facing a historic crossroads as it parts ways with the European Union after nearly 50 years on Friday night.

US President Donald Trump has long backed Brexit and trusted the ability of Prime Minister Boris Johnson -- a headline-grabbing leader he once compared to himself -- to set Britain on a more independent and pro-American course.

Yet Johnson has confounded expectations and sided with Europe on everything from the need to save the Iran nuclear deal that Trump abandoned to the importance of China's Huawei tech giant to speedy 5G mobile.

Pompeo said Britain's decision to ignore months of US warnings and give Huawei a limited but still leading role Tuesday was "something we'll have a conversation about".

"We will make sure that when American information passes across a network we are confident that that network is a trusted one," Pompeo told reporters on his way to London.

"It's a little unclear precisely what they're going to permit and not permit so we need to take a little bit of time to evaluate that."

Boris Johnson and Mike Pompeo
US Secretary of State Mike Pompeo's meetings with British Prime Minister Boris Johnson threatens to become a damage limitation exercise for the "special relationship". Photo: AFP / JIM WATSON

Johnson told parliament his government will "do absolutely nothing to imperil our relationship with the US".

But a senior UK official conceded that Pompeo's private dinner Wednesday with Foreign Secretary Domic Raab and meeting with Johnson on Thursday in Downing Street might be tense.

"In any mature relationship there are issues on which we don't agree," the British official said. "We will have a candid conversation."

Some security analysts said Britain's exclusion of Huawei from "core" parts of the network may give Washington sufficient grounds to continue sharing intelligence and scale down the dispute.

"Essentially what they've done is limit Huawei from large parts of the network," said the Royal United Services Institute think-tank's cyber research chief James Sullivan.

Britain to exit from EU on Jan.31
Trump has been a big fan of both Johnson and Brexit. He dangled the promise of a new trade deal with Britain by as early as the end of the year. Photo: AFP / NICHOLAS KAMM

Pompeo noted the UK concession but said it was unclear how it was going to work.

"We have to see what that means from an execution and implementation perspective," Pompeo said.

Johnson also took sounded a positive note about a Middle East peace plan that the Palestinians rejected even before Trump unveiled it alongside Israeli Prime Minister Benjamin Netanyahu on Tuesday.

"No peace plan is perfect but this (one) has the merit of a two-state solution," Johnson said.

Both Johnson and Trump have incentives to preserve their "special relationship" in the post-Brexit age.

Johnson's government has touted Britain's ability to strike a lucrative trade deal with the United States as one of the big advantages of leaving the EU.

Britain will lose its privileged access to the single European market -- the world's largest and most important for UK trade -- once the post-Brexit transition window shuts at the end of the year.

And Trump could benefit politically from a trade deal with a major partner in the run-up to November's presidential election.

Trump pledged last month to push for a "massive" post-Brexit trade deal with Johnson.

The chances of one getting the Congress in the heat of the US election campaign appear remote.

But the two leaders could sign off on a bare-bones agreement that helps ease future talks.

Pompeo said London and Washington had "enormous trade issues" to discuss.

A deal is being complicated by Trump's unpopularity in Britain and domestic pressure on Johnson to stand up to the United States.

UK media took huge offence at Washington's refusal to extradite the wife of a US diplomat who is using the cover of diplomatic immunity to avoid prosecution over the death of a teenager in a road accident in England.

A US prosecutor's complaint Monday that Britain's Prince Andrew was stonewalling an FBI investigation into the late sex offender Jeffrey Epstein only added to the layers of tensions.

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