THERESA MAY's team are keeping a close eye on Boris Johnson who is feeling "so low" there are fears he could quit.
The Foreign Secretary is said to feel sidelined over Brexit talks and our divorce bill, The Times reported.
As the Tories gear up for their party conference later this month in Manchester, there are fears that he could use his key speech to cause trouble.
Relations between the pair have been frosty since he was appointed to the role last summer, with Boris making a number of slip-ups on the world stage.
Earlier this year he compared the French President to a World War II prison guard for giving the UK "punishment beatings", and he was also furiously slapped down after accusing Saudi Arabia of engaging in "proxy wars".
Even Mrs May herself mocked her then leadership rival last year, saying "the last time he did a deal with the Germans he came back with three nearly new water cannon!".
Since Mrs May's disastrous election campaign where she lost her majority, there have been rumblings about Tories set to oust her as PM.
But Boris leapt to her rescue and backed her on the night of the loss, a new book revealed last weekend.
He is said to have two red lines – the size of any exit payments and a limit of two years on a transitional deal.
But one cabinet minister thinks he is still the biggest threat to her long-term stability, the paper reported.
On a recent trip to Japan the PM said she was "not a quitter" and vowed to fight the next General Election too.
But Boris, who is popular among the Tory members and the general public, is still a concern for her.
One minister told The Times: “Boris does seem out of sorts. Whenever he gets into a car with acolytes like Jake Berry [an MP who backed him on his last leadership bid] we suspect something may be up, and we’ve noticed that recently.”
And another source said he was "so low" that he was on resignation watch with Downing Street.