"Crikey." That was the one-word reaction from a senior MI6 officer when told by a colleague in late 2001 that the Americans were serious about war in Iraq.
CIA officers also recall the shock of British counterparts. "I thought they would have a heart attack right there at the table," recalls Luis Rueda, head of the CIA's Iraq Operations Group. "If they weren't gentlemen, they would have reached across the table and slapped me.
The message soon reached Downing Street. It would be spies rather than diplomats who would deliver it.
"I was probably the first to say to the prime minister, 'Whether you like it or not, get your ducks in a row because it looks as though they're building up to an invasion," the then-head of MI6, Sir Richard Dearlove, and a frequent visitor to Washington, tells the BBC in a rare interview.
MI6 - the UK's foreign intelligence service - was about to become deeply embroiled in one of the most controversial and consequential episodes in its history.
For the US, the issue of weapons of mass destruction (WMDs), were secondary to a deeper drive to overthrow the Iraqi leader, Saddam Hussein. "We would have invaded Iraq if Saddam Hussein had a rubber band and a paperclip," says Mr Rueda. "We would have said, 'Oh, he will take your eye out.'"
It has sometimes been alleged that the UK government made up the claims about WMDs. But ministers from that time say they had been assured by their own spies the weapons did exist.
"It's really important to understand the intelligence I was getting is what I was relying on, and I think I was entitled to rely on it," former Prime Minister Sir Tony Blair tells me. On the eve of the invasion he says he asked for - and was given - reassurances from the Joint Intelligence Committee. He declines to criticise the intelligence services for getting it wrong.
Other ministers say they had doubts at the time.
"On three occasions I questioned Richard Dearlove about the provenance of this intelligence," says the then foreign secretary, Jack Straw. "I just had an uneasy feeling about it. But Dearlove assured me on each occasion that these agents were reliable." However, Mr Straw says that it is ultimately for politicians to take responsibility, because they make the final decisions.
Asked if he looks back on Iraq as an intelligence failure, Sir Richard's answer is simple: "No." He still believes Iraq had some kind of weapons programme and that elements may have been moved over the border to Syria.
Some inside MI6 say they also had concerns. "At the time I felt what we were doing was wrong," says one officer who worked on Iraq, who has never spoken before and asked to remain anonymous.
"There was no new or credible intelligence or assessment which suggested that Iraq had restarted WMD programmes and that they posed an imminent threat," says the former officer, speaking of the period of early 2002. "I think from the government's point of view it was the only thing they could find…. WMD was the only peg they could hang the legality on."
Existing intelligence in the spring of 2002 was patchy. MI6's long-standing agents in Iraq had little or no information about WMDs, and there was a desperate hunt for fresh intelligence from new sources to bolster the case, particularly when a dossier was planned for September.
Another insider recalls decoding a message saying there was "no more important role" for the intelligence service than persuading the British public of the case for action. They say that questions were raised if this was appropriate, and the message was deleted.
Sir Richard dismisses accusations that he got too close to Downing Street as "ridiculous" but will not comment on the details of the case or specific sources. But in the coming months, this new source never delivered and was ultimately deemed to have been making it up, other intelligence sources say. Quality control was breaking down, they argue.
It was likely some of the new sources were making information up for money or because they wanted to see Saddam overthrown. In January 2003, I met a defector from Saddam's intelligence service in Jordan. He claimed to have been involved in developing mobile laboratories to work on biological weapons, out of sight from UN inspectors.
His claims made it into US Secretary of State Colin Powell's presentation to the UN in February 2003, even though some within the US government had already issued a "burn notice", saying that the information was not to be trusted. Another source codenamed "Curveball", whom the US and UK relied on, was also making up details about the labs.
Saddam had ordered the destruction of much of his WMD programme in the early 1990s after the first Gulf War in the hope of getting a clean bill of health from UN weapons inspectors, one of Iraq's top scientists later told me. The Iraqi leader may have hoped to restart the programmes at a later point. But he had destroyed everything secretly, partly to maintain the bluff that he might still have something he could use against neighbouring Iran, with whom he had just fought a war. So when Iraq was later asked by UN inspectors to prove it had destroyed everything, it could not.
One Iraqi scientist later revealed that they had disposed of a deadly compound that western intelligence agencies said was unaccounted for, by pouring it into the ground. But they had done so near one of Saddam's palaces, and they feared that owning up to this fact would have got them killed by the Iraqi leader. The result of all this was that Iraq could never really prove that it no longer had weapons.
By the end of 2002, UN inspectors were back in Iraq looking for WMDs. Some of those inspectors, speaking to the BBC for the first time, can recall looking at sites where intelligence tip-offs from the West suggested mobile labs might be based. They found only what one calls a "glorified ice cream truck" covered in cobwebs.
The public at the time never learned that as war approached, with sources failing to deliver and inspectors drawing a blank, there were concerns. "Panicky" is how one insider describes it. "My future is in your hands," Mr Blair said, half-jestingly, to Sir Richard in January 2003, as the pressure was growing to find proof of WMDs.
The failure to find a "smoking gun" would not stop war in March 2003.
"I tried right until the last moment to avoid military action," Tony Blair tells the BBC. President George Bush, fearing his ally would lose a vote in parliament on the eve of war, did offer him in a video-call the opportunity to back out of the invasion and only be involved in the aftermath, but the prime minister turned it down.
He defended his decision both as a matter of principle in terms of the need to deal with Saddam Hussein, but also because of the need to maintain the UK's relationship with the US. "It would have had a significant impact on the relationship," he says, adding: "When I was prime minister, there was no doubt either under President Clinton or President Bush, who the American president picked up the phone to first. It was the British prime minister. Today we're out of Europe and would Joe Biden pick up the phone to Rishi Sunak first? I'm not sure."
But no WMD would be found afterwards either. "It all fell apart," one former MI6 officer says, recalling a post-war internal review of sources. And this would leave deep and lasting consequences for both spies and politicians.