US Secretary of State Mike Pompeo on Thursday called China's ruling Communist Party "the central threat of our times" that challenges Western principles.

"The Chinese Communist Party presents the central threat of our times," Washington's top diplomat said at a joint appearance with Britain's Foreign Secretary Dominic Raab.

He added that Western allies must "ensure that the next century is governed by these Western democratic principles".

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.