Donald Trump's furious administration has turned the screws on Boris Johnson as he decides whether to let a Chinese firm into the UK phone network.
It's thought the government will announce on Wednesday whether Huawei can build a new super-fast 5G system across the UK - despite security fears.
Britain is widely expected to allow the Chinese firm access. Yet that has infuriated the US President, who raised it in a phone call with Boris Johnson.
And last night US Secretary of State Mike Pompeo ramped up the pressure by warning it will hurt the UK's "sovereignty".
That warning could have vast implications because the US depends on UK data security for many of its 'Five Eyes' relationships to fight crime and foil terror.
Mr Pompeo- who will visit the UK and meet Boris Johnson this week - wrote on Twitter last night: "The UK has a momentous decision ahead on 5G.
"British MP Tom Tugendhat [who has warned against Huawei] gets it right: 'The truth is that only nations able to protect their data will be sovereign'.
He retweeted a comment by Mr Tugendhat, the chairman of the Commons Foreign Affairs Committee in the last parliament, in which the MP said: "Sovereignty means control of data as much as land.
"We need to decide what we're willing to invest in and who were willing to share our tech with.
"The real costs will come later if we get this wrong and allow Huawei to run 5G."
Mr Tugendhat called for a "cap" that reduces the rule of Huawei technology in the UK's and stops it being fully “built in” to network.
"It's no good taking back control from Brussels only to hand it over to Beijing. And I'm sure that's not what the Government would want to do," he told BBC Radio 4's Today programme.
A senior Trump administration official ramped up pressure further, telling Axios: “This is a highly consequential decision that the British prime minister’s going to be making.
"Not only in terms of their relationship with the United States, but first and foremost for their own citizens.
"People are going to be a bit shaken by the U.K.’s judgment if they make this decision."
Justice Secretary Robert Buckland was defiant last night - saying the decision would be taken in terms of the UK's national interest.
He told BBC Radio 4's Westminster Hour: "The decision we make will be based upon our own sovereign right to choose.
"It’s Britain that will have to live with the consequences of that.
"There are risks but we will make an informed decision based on the evidence and we will do so in an autonomous way.”
And the UK's former national security adviser Peter Ricketts, a crossbench peer, said the risk is being blown out of proportion.
"I'm quite sure, knowing the intelligence community well, that they won't be recommending to ministers, the course of action that is likely to mess up our intelligence relationship with the US or prejudice UK security," he said.
"I personally think we can find a solution which does allow them to have some role and which doesn't send the Americans off the other side of the diving board."
According to the Financial Times, the PM is looking at imposing a market share cap on Huawei to stop its dominance and entice other phone firms in.
But Shadow Brexit Secretary Keir Starmer demanded the PM make a statement to Parliament about the decision
He told Sky News: "I don't think the Americans should be dictating to us what arrangements we come to. But I am concerned that we shouldn't be giving too much power to the Chinese as well.
"The PM should make a statement about this this week.
"He's done a bit of a runner has Boris Johnson... he needs to come to Parliament, make a statement and face questions about this."
Tory MP Mr Tugendhat on Friday warned that Huawei has "consistently demonstrated" it operates under a different attitude to "rule of law and civil rights" overseas.
