Jackson Carlaw has resigned as the leader of the Scottish Conservatives after only six months in the job.
Mr Carlaw said he did not believe he was the best person to lead the case for Scotland remaining in the United Kingdom.
And he said he had therefore decided to stand down with immediate effect.
Mr Carlaw succeeded Ruth Davidson as the party’s leader in February after previously acting as her deputy.
He had also served as acting leader while Ms Davidson was off on maternity leave following the birth of her son, and again after her resignation in August of last year.
Prime Minister Boris Johnson said Mr Carlaw had been a “tremendous servant” to the party for more than four decades.
Mr Johnson added: “As an activist, deputy chairman, deputy leader and leader, he has given his all and deserves our thanks for his efforts.
“It is a mark of his commitment to the cause that he chooses to stand aside at this time and I offer my best wishes to him, Wynne and the family.”
In a statement, Mr Carlaw said he been “thinking hard” about his role as party leader over the summer months.
Mr Carlaw said: “Nothing is more important to me than making the case for Scotland’s place in the United Kingdom.
“I believe the Scottish Conservative and Unionist party is the most important voice in Scotland for setting out that positive argument. I am clear that nothing must get in the way of doing so.
“In the last few weeks, I have reached a simple if painful conclusion – that I am not, in the present circumstances, the person best placed to lead that case over these next vital months in Scottish politics prior to the Holyrood elections.”
His resignation was announced just hours after Mr Carlaw faced First Minister Nicola Sturgeon in her weekly question session in the Scottish Parliament.
Jackson Carlaw’s resignation comes completely out of the blue and has taken many in Scottish politics by surprise.
The Eastwood MSP took over as interim leader one year ago yesterday, when Ruth Davidson resigned, but thanks to Brexit and December’s general election the formal leadership contest did not conclude until February.
This means as recently as six months ago, Mr Carlaw was arguing in his typically energetic style that he was the best person to lead the Scottish Tories.
Now, somewhat abruptly, he has decided he wasn’t after all. Yes, our world has changed enormously in the intervening time, but it might be fair to wonder what has happened to inspire such speedy self-reflection.
With the Holyrood elections now months away and the SNP riding high in the polls, this comes at an acutely difficult moment for Mr Carlaw’s party.
They will have to move quickly if the next leader is to have their feet properly under the table by the time they have to face the electorate as a whole.
The Scottish Conservatives are currently the second largest party at Holyrood, but are facing a battle to prevent the pro-independence SNP winning a majority in next year’s election.
Mr Carlaw said he was confident that he was leaving the party “in good heart and, crucially, with time to elect a new leader so he or she can prepare for the elections next year.”
He said the party would continue to “unequivocally speak up for all those Scots who do not want to go back to more division, but instead want our country to move on, as part of the United Kingdom, able to rise to the challenges of the future.”
And he pledged: “I will fight that cause hard for these next few vital months as a loyal member of my party.”
Ms Sturgeon tweeted her best wishes to Mr Carlaw, saying that “leadership is a tough business and I’m sure his decision wasn’t easy.”
She added: “We’ve crossed swords politically on many occasions, but worked constructively on some issues too – he has, eg, been a strong voice for women suffering mesh complications.”