Ruth Davidson is expected to quit as leader of the Scottish Conservative party because of her irreconcilable differences with Boris Johnson over Brexit and the pressures of motherhood.

Scottish Tory sources said an announcement was imminent. Davidson was “considering her position”, one source added, although he stressed it was not connected to Johnson’s decision to prorogue parliament.

“Ruth will make her decision clear in due course, and there will be no further comment this evening,” her spokesman said on Wednesday.

She made abundantly clear during the Tory leadership campaign that she did not believe Johnson was the right UK party leader, with some of her close allies describing Johnson as a “disaster” for the Scottish Tories.

One of Johnson’s first acts as prime minister was to sack her close friend and ally David Mundell, who served as Scottish secretary for five years, against her advice.

Davidson was reported to be “livid” at Mundell’s sacking and went on to issue a defiant challenge to Johnson on the eve of his first visit to Scotland as prime minister, pledging she would refuse to back a no-deal Brexit. In her regular column for the Scottish Mail on Sunday she also promised that she “will not be backward in challenging Mr Johnson’s government where I think they are getting it wrong”.

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A prominent remain campaigner, Davidson famously clashed with Johnson in a televised debate during the European referendum campaign in 2016, accusing his leave campaign of telling “untruths” about the EU.

It remains unclear whether Davidson will stay on as an MSP. She won the constituency of Edinburgh Central unexpectedly in the 2016 Holyrood byelection, largely thanks to her popularity and her appeal to latent Tory voters.

However, she won with a slender lead of just 610 votes over the Scottish National party, which would be very confident of winning the seat in a byelection where Brexit and the prospect of Scottish independence were the defining issues.

Holyrood is already braced for the results of Thursday’s Scottish parliamentary byelection in Shetland, where the SNP hopes to win a seat held comfortably by the Liberal Democrats for 20 years. A win for the SNP in Shetland would leave Nicola Sturgeon’s party with 63 seats, tied with all other opposition parties. If the SNP won another byelection in Edinburgh Central, the first minister would win a parliamentary majority.

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Davidson, the first openly lesbian party leader anywhere in the UK, gave birth to her first child, conceived with her partner, Jen, following IVF treatment, last October. As a state-educated woman in a party overwhelmingly controlled by privately schooled men, she has delivered previously unthinkable electoral success for the Scottish Tories at both Holyrood and Westminster.

Last September, Davidson published her first book, Yes She Can, a collection of interviews with “mould-breaking women”, in which she revealed that she was diagnosed with clinical depression as a teenager and discussed the continuing management of her mental health.

Davidson attracted criticism from opposition politicians, including Sturgeon, for not giving media interviews on Wednesday following the prorogation announcement.

Davidson’s allies would not be drawn to speak publicly before her own statement on Thursday, and privately only expressed admiration for her leadership and sadness at her decision.

Several years ago Davidson was touted as a potential leader of the UK Tories and a future prime minister, although she repeatedly rebuffed suggestions she would look for a safe Westminster seat.

In June a YouGov poll of Tory members suggested nearly two-thirds would rather Scotland left the UK than Brexit not take place. Davidson said the party’s members needed to “take a long, hard look at themselves”.