Ruth Davidson is on the verge of quitting as leader of the Scottish Conservatives, BBC Scotland understands.
Ms Davidson has been deeply unhappy with the prime minister's pursuit of a possible no-deal Brexit.
She is said to be considering her position, with a statement on her future likely to be made on Thursday.
Ms Davidson, who gave birth to a son in October of last year, has been leader of the party since November 2011.
Sources have told the BBC that Ms Davidson's decision was based on a mixture of personal and political factors which have been building over recent months.
And they insisted it was not a direct result of the announcement by Mr Johnson that he wants to suspend parliament in September - only a few weeks before the Brexit deadline of 31 October.
The Scottish Daily Mail on Thursday quoted her as saying her time in frontline politics has taken a "toll" on her family and friends as well as herself.
She said: "Those of us who are lucky enough to serve in political leadership accept the toll it takes but there's always a part of us which can never accept the effect it has on family and friends too.
"As we go forward to an incredibly turbulent time in UK and Scottish politics, I want to make clear the Scottish Conservative and Unionist Party is in great shape and will continue to hold the SNP to account
"Scotland's best interests will always be as part of the UK and everyone in the party - from the leader to our activists - are determined to make that case every day from now until the general election."
BBC Scotland's political editor, Brian Taylor, said that he believed Ms Davidson has made her mind up to stand down.
He added: "She has been eight years as Conservative leader, during most of which she has driven the party and the Conservative cause forward. She was talked of at various points as being a potential prime minister and said herself the ambition was to be first minister.
"But it would seem she is, amid the turmoil of Brexit, resigning from both of those options."
The Scottish Sun, which broke the story, said it had been told by a senior Conservative source that juggling motherhood with politics had "taken its toll" on Ms Davidson, as had the "current political climate, where she's found herself at increasing odds with the new leadership in London".
It has been reported that she is likely to continue serving as the MSP for Edinburgh Central.
Ms Davidson backed Remain ahead of the EU referendum, and famously clashed with Mr Johnson in a live TV debate at Wembley before the vote - when she accused the Leave side of telling "untruths".
She publicly backed opponents of Mr Johnson in the Conservative leadership contest, and has previously said she would not support a no-deal Brexit.
Ms Davidson was also said to have been angered by Mr Johnson's decision to sack David Mundell - a close political ally of hers - as Scottish secretary and replace him with Alister Jack.
The 40-year-old was widely credited with turning around the fortunes of the Scottish Conservatives, with the party more than doubling its number of seats at Holyrood in the 2016 election - making it the second biggest party behind the SNP - and increasing its MPs from one to 13 in the 2017 general election.
That relative success had seen her tipped as a future UK Tory leader, but she insisted she had no interest in the job - and that she was focused only on replacing Nicola Sturgeon as first minister of Scotland at the next Holyrood election in 2021.
When she returned to the Scottish Parliament from maternity leave in May, she told BBC Scotland that it would be "tough to tear myself away from the wee man" but that "having a child and coming back to work is a part that lots of women play".
Reacting to the news that Ms Davidson may quit, Scottish Labour Leader Richard Leonard said: "If the rumours are true then the loss of Ruth Davidson as leader of the Scottish Conservatives will be a real blow to her party.
"This shows that even within his own ranks, Boris Johnson is already losing support and credibility. Scotland and the UK needs a general election as quickly as possible to oust him from Downing Street and elect a Labour government."
Scottish Liberal Democrat leader Willie Rennie said: "It's no surprise Ruth Davidson has run out of reasons to stand by this dangerous and power hungry prime minister.
"There must be lots of other like-minded conservatives who are horrified at this blatant abuse of power and can't stomach being party to this destructive agenda any longer. They should take this as the signal to abandon ship."
Scottish Greens co-leader Patrick Harvie said: "It seems clear that Ruth Davidson and several of her senior colleagues at Holyrood are every bit as disturbed as the rest of the country by Brexit itself, and by the dangerous No Deal agenda of Boris Johnson.
"Ruth may be have unable to foster any kind of rational politics inside the Conservative Party, but she is at least due some credit now for ending the pretence and resigning as party leader, and at a personal level I wish her well for the future."