MI6 chiefs are secretly battling Donald Trump to stop him publishing classified information linked to the Russian election meddling investigation.
The UK is warning that the US president would undermine intelligence gathering if he releases pages of an FBI application to wiretap one of his former campaign advisers.
However Trump allies are fighting back, demanding transparency and asking why Britain would oppose the move unless it had something to hide.
It forces the spotlight on whether the UK played a role in the FBI's investigation launched before the 2016 presidential election into Trump campaign ties to the Kremlin.
The Telegraph has talked to more than a dozen UK and US officials, including in American intelligence, who have revealed details about the row.
British spy chiefs have “genuine concern” about sources being exposed if classified parts of the wiretap request were made public, according to figures familiar with discussions.
“It boils down to the exposure of people”, said one US intelligence official, adding: “We don’t want to reveal sources and methods.” US intelligence shares the concerns of the UK.
Another said Britain feared setting a dangerous “precedent” which could make people less likely to share information, knowing that it could one day become public.
The current row is deemed so politically sensitive that staff at the British embassy in Washington DC have been barred from discussing it with journalists.
Theresa May, who already has a testing relationship with Mr Trump, has also been kept at arms-length and is understood to have not raised the issue directly with the US president.
Trump's two-year headache
The row is about an FBI request to wiretap Carter Page, a former Trump campaign foreign policy adviser, which was made in October 2016 - the month before the US election.
The FBI said it had suspicions Mr Page was being targeted for recruitment by the Russian government and cited classified intelligence to make its case.
The agency was granted approval for 90 days of surveillance by a secret court established by the Foreign Intelligence Surveillance Act [Fisa]. It was renewed a number of times.
Republican allies of Mr Trump in the US Congress have pounced on the application, claiming the FBI failed to follow due process and presented information in a flawed way. A heavily redacted version of the documents has already been released.
International intelligence concerns
Mr Trump wants to declassify 21 pages from one of the applications. He announced the move in September, then backtracked, then this month said he was "very seriously" considering it again. Both Britain and Australia are understood to be opposing the move.
Memos detailing alleged ties between Mr Trump and Russia compiled by Christopher Steele, a former MI6 officer, were cited in the application, which could explain some of the British concern.
Numerous spokesmen for UK and US government bodies declined to comment. Mr Page has denied being a foreign agent for Russia.
The row comes as UK intelligence agencies are increasingly dragged into a heated and partisan battle in Washington DC over the origins of the Russian investigation.
Mr Trump’s allies and former advisers are raising questions about the UK’s role in the start of the probe, given many of the key figures and meetings were located in Britain.
Ex-adviser renews accusations
George Papadopoulos, a former Trump campaign adviser, has publicly claimed he was targeted by UK spies and told The Telegraph that he is demanding transparency.
Republicans are attempting to protect the US president by suggesting the Russia investigation, which continues to this day, was invalid from the start.
The probe, now overseen by special counsel Robert Mueller, is looking into whether Trump campaign figures conspired with the Kremlin and whether the president obstructed justice.
By suggesting the investigation was created by shadowy intelligence figures who wanted to thwart Mr Trump’s candidacy from the start, Republicans are making it easier for the eventual findings to be waved away.
However a result of the attack line is that Britain’s spy agencies are being included in claims of “deep state” opposition to Mr Trump. It risks inflaming UK-US tensions at a time when Britain wants to deepen ties with America as it leaves the European Union.
The row over the Page wiretap application is not the only manifestation of the tensions.
Mr Papadopoulos, a former Trump campaign adviser sentenced to 14 days in prison for lying about his conversations with Russian-linked figures, has begun publicly pointing a finger of blame at Britain.
In April 2016, Mr Papadopoulos was told by Joseph Mifsud, an academic allegedly tied to Russia, that the Kremlin had damaging emails about Hillary Clinton, the Democratic presidential nominee, before the documents had been made public.
In May 2016, Mr Papadopoulos reportedly mentioned that fact over a drink with Alexander Downer, the Australian high commissioner in London - a boast which triggered the FBI to start ‘Operation Crossfire Hurricane’, which would eventually become the Russia probe.
In the last two-and-a bit months Mr Papadopoulos has been interviewed at least six times on Fox News - Mr Trump’s favourite cable news channel - and pushed the idea that Western intelligence set him up.
He has suggested he was “lured” to London “so that the British would spy on me” and was targeted by a “plot by Western intelligence”. The Fox News presenters have often echoed the suggestions.
Mr Papadopoulos told The Telegraph: “The British Government has a lot of explaining to do. It’s in their interest to be transparent. Why was the British intelligence apparatus weaponised against Trump and his advisers?”
Other Trump allies are pushing similar claims. One former top White House adviser to Mr Trump made similar insinuations, telling this newspaper: “You know the Brits are up to their neck."
The source added on the Page wiretap application: “I think that stuff is going to implicate MI5 and MI6 in a bunch of activities they don't want to be implicated in, along with FBI, counter-terrorism and the CIA."
Republican claims rebuked
One former UK official warned that many of the attacks seem to originate from right-wing internet forums, such as 4chan. The claims must be treated with suspicion given they are often cited without hard evidence and bring a political benefit to the White House.
GCHQ, Britain’s secret listening post, issued a rare on-record statement last year denying a suggestion quoted by Sean Spicer, then the White House press secretary, that it had helped wiretap Trump Towers. A GCHQ spokesman called the claim “nonsense” and “utterly ridiculous”.
Tony Blair, the former prime minister, also had to publicly deny a suggestion he told Jared Kushner, Mr Trump’s son-in-law, that UK intelligence agencies may have been involved in surveillance of the Trump campaign.
The claim was made in Michael Wolff’s book Fire and Fury. Mr Blair said the suggestion was a “complete fabrication” from “beginning to end”.
With Mr Mueller's probe appearing to be nearing its end, UK officials are braced for Trump allies to ramp up the claims as they look to protect the president from any damaging revelations.