Ruth Davidson is poised to quit as leader of the Scottish Conservatives.
The former journalist, who took over the top job in the party almost eight years ago, will "make her position clear in due course", a spokesman said.
But it is expected she will step down on Thursday - with her resignation coming in the aftermath of Prime Minister Boris Johnson announcing plans to temporarily suspend Parliament.
Ms Davidson, who campaigned for Remain in the 2016 European referendum, had clashed with Mr Johnson over the issue of the UK's future membership of the EU.
The Scottish Tory - who was touted as a future leader of the UK Conservatives until she explicitly ruled out taking on the post - went up against Mr Johnson in a televised referendum debate at Wembley Stadium in 2016.
She has been a vocal critic of Mr Johnson, and during the race to succeed Theresa May Ms Davidson backed first Sajid Javid and then Jeremy Hunt over the eventual winner.
A spokesman for the Scottish Conservatives said on Wednesday: "Ruth will make her position clear in due course. There will be no further comment tonight."
However he stated: "This decision is not connected to today's Brexit developments in any way."
Ms Davidson, who is engaged to partner Jen Wilson, became a mother for the first time in October 2018 when she gave birth to their son Finn after IVF treatment.
She stepped out of the political frontline to go on maternity leave, with her deputy Jackson Carlaw standing in as interim leader.
She made her comeback speech to the Scottish Conservative conference in Aberdeen in May this year.
Her leadership has transformed the fortunes of the Conservatives north of the border after she took on the job in November 2011 - just six months after she became an MSP.
Under her, the Scottish Conservatives won a record 31 seats at Holyrood in the 2016 election, with the Tories ousting Labour from second place.
She cemented this result with a strong performance in the 2017 general election, when the party returned 13 MPs from north of the border - a result that helped keep Mrs May in Downing Street.
After Mr Johnson took over as PM, Ms Davidson lobbied to keep David Mundell - who she had previously described as her "work husband" - in post as Scottish Secretary, only for him to be sacked from the job.
Scottish Labour leader Richard Leonard said the loss of Ms Davidson would be a "real blow to her party".
The Labour MSP added: "This shows that even within his own ranks, Boris Johnson is already losing support and credibility."
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."
Ian Murray, the Labour MP for Edinburgh South, said: "Ruth Davidson is a formidable politician and her departure will be deeply felt by her party.
"She campaigned tirelessly for a Remain vote in the EU referendum, but the Tories under Boris Johnson have become a hard Brexit party, so it is not surprising that it has come to this."
Pamela Nash, chief executive of the campaign group Scotland in Union, said: "Ruth's leadership will be remembered for her relentless work making the case for Scotland to remain in the UK.
"Her departure as leader would be sad news for Scotland, which needs vocal champions to stand against the SNP's divisive nationalism."
