Theresa May allegedly ordered some information be withheld from Boris Johnson when he entered office in 2016 over fears he could not be trusted.
The Sun claimed that nerves were sparked when Johnson was accused of revealing classified information by mistake.
A source told the paper: “The PM didn’t think Boris could be trusted because he had a loose tongue. He made the agencies anxious. He wasn’t told everything because of that.
“Pre-meetings would be arranged without his knowledge before he’d come over to No 10.
“They probably both share the blame for the situation and it was a clash of their worst traits – Boris is a big mouth and Theresa can be a paranoid control freak.”
A source told the BBC that Johnson was aware of the decision at the time and “very unhappy” about it.
But appearing at a Conservative leadership hustings in Darlington the Tory leadership front-runner said the reports were “not true”, adding he was “extremely dubious about the provenance of this story”.
He said: “I am sure that the Prime Minister would not comment on intelligence matters either so I am extremely dubious about the provenance of this story.
“It’s not true and I don’t comment on intelligence matters.”
Downing Street said it did not comment on intelligence matters, but asked if May trusted Johnson, a Number 10 spokeswoman said: “Yes. It’s a matter of fact that it was the PM’s own decision to appoint Boris Johnson as foreign secretary in full knowledge of all responsibilities that that job involves.”
Foreign secretary Jeremy Hunt refused to be drawn on the reports, telling the hustings: “We do have a rule that we never comment on intelligence matters and that would include comments about my predecessor and comments about myself.
“We have the finest intelligence services in the world in this country – that does depend on some discretion by the Foreign Secretary so I’m not going to breach that now.”