The married 66-year-old was later cleared of wrongdoing following a probe by the Department of Justice.Suspicion among US officials was reportedly raised after a series of articles by Miss Kosinski, 46, detailing highly-sensitive information about the Trump administration.
Lord Darroch was reportedly suspected of tipping off Miss Kosinski about plans to make CIA director Mike Pompeo the Secretary of State, which did later happen.
Miss Kosinski is also said to have learned about Mr Pompeo’s plans for a trip to North Korea to push for the return of three US citizens. No story was reported but the White House became more interested in Miss Kosinski.
She also apparently raised eyebrows when tipped off about US plans for tough sanctions against Russia.
The classified proposals had reportedly been discussed in private with allies in France, Germany and the UK before being announced on April 6.
A source told The Sun that Miss Kosinski ‘knew about these sanctions before they were announced’. She also caused concern after learning about a behind-closed-doors feud between Donald Trump and the US ambassador to the United Nations, Nikki Haley.
Lord Darroch has been married for 42 years and has two grown-up children.
He has vehemently denied any impropriety and suggestions that he leaked the stories. His affair with the journalist allegedly began at the start of 2018.
The Sun said it had seen ‘intimate’ messages between the pair detailing their plans to meet up.
US authorities probed the alleged relationship in detail over fears it could open up Lord Darroch to blackmail by hostile states.
Miss Kosinski, who is also married, left her post at CNN just five months after the envoy left his top job, according to her LinkedIn profile.
Lord Darroch quit his Washington post in July last year after leaked cables revealed he had branded Mr Trump ‘incompetent’.
He also claimed the President ‘radiates insecurity’, telling British officials that they need to ‘start praising him for something he’s done’ when they meet him.
Officials are likely to be shocked by the allegation one of Britain’s most senior diplomats had an affair with a journalist who could have published material linked to him.
Miss Kosinski was approached for comment last night.