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Reuters
US Secretary of State Mike Pompeo is expected to warn the UK about allowing Chinese firm Huawei access to critical infrastructure on a visit to London.
Mr Pompeo will hold talks with Theresa May and make a speech on foreign policy ahead of US President Donald Trump's state visit next month.
The US is reported to be alarmed at Huawei's possible involvement in building the UK's 5G mobile network.
The UK government has insisted no final decision has been made.
The issue caused a major political row last week after defence secretary Gavin Williamson was sacked from cabinet after leaks of discussions from a National Security Council meeting.
The BBC's diplomatic correspondent James Landale said Mr Pompeo was expected to warn about the risks of the UK forming a partnership with the Chinese firm.
He said the top diplomat would also warn that the US might be forced to reduce its presence in the UK if the Chinese firm had access to critical data.
The US, which is the UK's leading intelligence partner, has said it will not give Huawei, which its critics say is an arm of the Chinese Communist Party, any 5G contracts, amid concerns it could be used to spy on the US.
Press reports suggesting the UK had agreed to allow the Chinese firm to participate in non-core elements of the 5G program program led to Mr Williamson's dismissal - although he has insisted that he was not the source of the leaks.
On his one-day visit, Mr Pompeo will also have a working lunch with Foreign Secretary Jeremy Hunt and take part in a roundtable discussion on religious freedom chaired by the Archbishop of Canterbury.
James Landale said British and US policy towards Iran would also be discussed after Tehran announced it was pulling out of key commitments under the 2015 international nuclear deal.
Last year, President Trump withdrew the US from the multinational accord designed to curb Iran's nuclear ambitions.
The UK, France, China, Germany and Russia remain signed up to it.