The Sun understands married Lord Darroch’s passionate affair with CNN diplomatic correspondent Michelle Kosinski went on for at least several months.
The Department of Justice probe into the 66-year-old diplomat is thought to be the first into a serving British ambassador in Washington.
It was launched after top US government officials were “astonished” by a series of articles by Kosinski, 46,
One story about plans for a top–secret visit to North Korea by Secretary of State Mike Pompeo sparked horror, with US security interests said to be at risk.
Lord Darroch, married for 42 years and with two grown-up children, is believed to have fiercely denied any impropriety and any suggestion he was the leaker — and he was eventually cleared.
Ironically he quit his post in July last year after his own diplomatic cables were leaked, showing he had branded President Donald Trump “inept” and “incompetent”.
Kosinski’s public LinkedIn profile shows she left her post at CNN five months later.
A security source said: “The leaks of the cables last year were obviously deeply embarrassing and left Darroch no option but to resign once President Trump made his feelings clear.
“But for the US to investigate the most senior diplomat of its greatest ally on suspicion of leaking secret information is almost unprecedented.
“Although the investigation found nothing over Darroch, the fact it happened is extraordinary.”
The Sun understands married Lord Darroch’s passionate affair with Michelle Kosinski went on for at least several months
Lord Darroch, from South Stanley, Co Durham, had joined the Foreign and Commonwealth Office in 1977 and tied the knot the following year.
He was Tony Blair’s European Union advisor and later National Security Advisor to David Cameron.
His NSA job gave him wide- ranging access to top-secret intelligence about the rise of IS in Syria, Russian aggression and Iran’s nuclear programme. In 2016, as Sir Kim Darroch, he landed the coveted post of UK ambassador to the US.
But as the Darrochs approached their ruby wedding anniversary, the ambassador was allegedly engaged in a special relationship of his own.
The Sun understands he had begun his affair with Kosinski, who is also married, by the beginning of 2018.
They swapped intimate messages, seen by The Sun, detailing plans for secret hook-ups.
But as their relationship developed, Kosinski’s scoops began to attract the attention of senior White House officials.
The experienced reporter was far from a star name in the US media, with Washington insiders describing her as “not well-connected”.
Kosinski left her post at CNN five months after Lord Darroch resigned as the UK's US amassador.
But sources say she suddenly began to steal a march on her rivals, with the inside track on the Trump presidency’s most sensitive plans.
In February 2018 Lord Darroch is understood to have been suspected of tipping off Kosinski that CIA Director Mike Pompeo had told his staff he would soon become Secretary of State.
He was duly sworn into the job, equivalent to Britain’s Foreign Secretary, in April.
Days later Kosinski allegedly learned Pompeo was secretly planning a hugely delicate trip to North Korea to lobby for the safe return of three US citizens.
A source told The Sun: “It was believed that no one on the US side would leak this information.
“It was critical for the American officials that this information did not get out. If it did, the North Koreans could have been spooked and the three Americans may never have been returned.”
No story ultimately ran. Pompeo’s trip went ahead in May and the US citizens were safely freed.
But by now, the White House was taking more than a keen interest in Kosinski’s work.
In March 2018, Britain was rocked when ex-Russian double agent Sergei Skripal and his daughter Yulia were poisoned with deadly novichok near their home in Salisbury, Wilts.
The assassination attempt was quickly identified as the work of Russian hitmen and sparked global fury.
The UK expelled 23 Russian diplomats. Other countries followed suit, but the US was still carefully weighing up its options.
Six days later, Kosinski reported that America was thinking of also expelling Russian diplomats — but warned that it was seen internally as a drastic move.
She later revealed “new and exclusive” details. Citing “a senior foreign diplomat”, she claimed European allies were getting “optimistic signals”.
Less than two weeks later Kosinski again raised eyebrows after apparently being tipped off about US plans for tough sanctions against Russia.
Sources say the classified proposals had been discussed in private only with allies France, Germany and the UK before they were formally announced on April 6.
A former US official said: “Kosinski knew about these sanctions before they were announced.”
With concerns growing, Kosinski set off White House alarm bells for a fourth time after learning of a private spat between Trump and Nikki Haley, at the time the US Ambassador to the UN.
Sources say it was then that officials began seriously looking into Kosinski for evidence she was benefiting from leaks at the heart of Trump’s administration.
Other posts by her revealed inside details of Mr Trump’s rare apology to ex-PM Theresa May after he gave an exclusive interview to The Sun saying she had “wrecked Brexit”.
And then, in a sensational twist, sources say US authorities uncovered Kosinski’s affair with Lord Darroch.
The relationship, which security sources say left Lord Darroch open to the threat of blackmail by a hostile foreign power, began to be examined in detail.
In the end Lord Darroch was cleared by the probe.
No action was formally taken against him but the fact that he was investigated will send shock waves through the diplomatic world.
After the diplomat’s cables were leaked, President Trump told reporters: “The ambassador has not served the UK well.”
He mysteriously added: “I can say things about him but I won’t bother.”
A former senior US official said Mr Trump was referring to the investigation and knew of his affair with Kosinski.
They added: “The President never holds back, but out of respect to the UK he did on this occasion.”
Days later Lord Darroch quit. Since then, he has been ennobled and published his memoirs called Collateral Damage — a thinly veiled reference to his fall from grace.
In his book he talks about his fury at the leaker of his diplomatic cables and how he wished he could meet them in “a prison visiting room”.