The White House has launched a major review of whether spy planes, intelligence officials and other US assets should be withdrawn from Britain after Downing Street agreed to let Huawei help build its 5G network.
Sources, including current US and UK officials, have revealed the process - not yet made public - is under way, threatening the "special relationship".
Every military and intelligence asset the Americans have in Britain is being assessed to understand the knock-on implications of letting Huawei, the Chinese tech giant, construct part of the new wireless network.
Whether highly classified missions increasingly should be carried out from countries other than Britain due to confidentiality fears is also being considered, This could see US agents being redeployed.
One former official who only recently left the White House's National Security Council (NSC), which is leading the review, said it was "likely" some assets would be removed from Britain.
"This was not a bluff. You cannot mitigate the danger Boris Johnson is exposing the UK to by letting Huawei into the network," the source said.
"This is the White House saying, 'OK, if they're going to go down this path and put themselves at risk, then how do we protect ourselves?'"
The review marks a significant escalation in the Huawei row, with the US now going beyond words of warning, and taking concrete steps that could end up harming military and intelligence ties.
The UK has maintained that giving Huawei limited access will not compromise its 5G network.
The review comes as US President Donald Trump takes an increasingly confrontational approach towards Beijing. He has blamed China for not doing more to stop the coronavirus outbreak when it first emerged there.
Mr Johnson announced in January that Huawei would be allowed to build some of Britain's 5G network - defying sustained lobbying against that by the Trump administration.
But he set restrictions, barring Huawei from "core" parts of the network, such as near military facilities and nuclear sites, and capping its share of non-sensitive parts to 35pc.
The Trump administration has long maintained that letting Huawei build any part of the 5G network would effectively give access to the Chinese government.
The totality of the review means everything from the more than 10,000 US military personnel in Britain to scores of military vehicles will be reviewed, not to mention intelligence operations.
Sources familiar with the review questioned whether US agents who carry out secret missions in Britain using personal phones and other internet-connected devices could really keep their messages safe.
Some Republicans see a pulling back in Britain as matching their belief in the need for a US military refocusing on Asia.
"Britain is forcing us into a corner to make decisions and ponder consequences we don't want to make or ponder. We would rather the special relationship be renewed and revitalised, but it is difficult to do when genuine security interests were discarded," a Republican congressional adviser said.
The British Defence Secretary was not informed of the review when he visited Washington in March.
Robert O'Brien, the US national security adviser, said that UK-US intelligence co-operation would continue, but added in a joke now pointed to by Trump administration insiders, it may have to be done with "carrier pigeons". No 10 and the UK's Ministry of Defence declined to comment. (© Daily Telegraph, London)