Douglas Ross has said the Conservatives cannot keep on losing elections, as pressure continues to mount on Boris Johnson after two by-election defeats in England.
The Conservatives lost to the Liberal Democrats in Tiverton and Honiton in Devon.
And they were also beaten by Labour in the Wakefield, a constituency in West Yorkshire.
It came after bruising results for the Conservatives in the local elections across the UK in May.
The double by-election losses led to the resignation of Oliver Dowden as chair of the Conservatives.
It has also raised further speculation that backbench Tory MPs could make a fresh push for Johnson to step down as leader.
However, during a trip to Rwanda, Johnson told reporters that he is “actively thinking” about a third term in office, suggesting he could remain in place into the mid-2030s.
Speaking to the BBC on Sunday, Scottish Conservative leader Ross indicated that there is “growing frustration” within the party at Westminster.
“I think Oliver Dowden’s resignation shows that these are very disappointing results,” he said.
“I never want to see the Conservatives losing seats at all, but both results are very difficult for us and the Prime Minister and the party have to reflect on that.
“And as Oliver Dowden said, it cannot be business as usual.”
The Scottish Conservative leader reiterated his stance that Johnson should not remain as Prime Minister.
He said: “I made that clear three weeks ago when in the vote of no confidence I came to the conclusion that I didn’t have confidence in the Prime Minister.
“And I was joined by 147 other Conservative MPs who made that point, 40% of the parliamentary party didn’t have confidence in the Prime Minister.
“So I think that is a growing frustration both within the parliamentary party and as we’ve seen in these by-elections within the public as well.”
Ross explained that the no confidence vote in the Prime Minister did not take place at a time he would have chosen.
However, he outlined the need for Johnson to now reflect on the results, despite having won the confidence vote in his leadership.
“As I said at the time of the no confidence vote, the timing of it was not of my choosing,” he said.
“I didn’t put in a letter to the 1922 Committee because I thought the timing was wrong.
“However, under the current rules, we have an election every year potentially in terms of no confidence in the Prime Minister.
“And when that was put in front of me, because 15% or more of my parliamentary colleagues put in a letter to the 1922 Committee, I had to vote one way or another.
“And I had to vote that I didn’t have confidence in the Prime Minister and that does seem to be reflected not just in these by-election results, but what we saw in local government results across the UK in May as well.”
The Conservative MP for Moray said that it is not a situation that is “done and dusted” after the Prime Minister survived the vote of no confidence.
And he pointed to Johnson’s predecessor in office as evidence that winning such a vote does not guarantee a longer term future.
“We know that Theresa May had more support from her parliamentary colleagues when she faced a vote of no confidence and she resigned just a few months later,” said Ross.
“So, it’s not just a situation where that vote is done and dusted and everyone moves on.
“Because clearly the public, and these two by-elections in particular, have not moved on and we’ve seen now the resignation of the party chair who sits, or did sit, in Cabinet up until Friday morning.
“So, this is a decision colleagues continue to look at and the Prime Minister must reflect on.”
Ross insisted that the party “cannot continue to keep on going losing election after election”, as he said those close to the Prime Minister must look at the situation.
He said: “I think clearly there will be discussions amongst colleagues. Oliver Dowden is a very well-respected colleague, he was a very good party chairman and secretary of state before that.
“And he will have spoken to others I’m sure since his resignation and particularly those close to the Prime Minister will have to look at what is the best situation for the country.
“And we cannot continue to keep on going losing election after election as we have in these two by-elections and the difficult results that the party faced in the local government elections in early May.”
Ross was also asked about speculation that the rules of the influential backbench 1922 Committee could be changed to allow another vote of no confidence to take place.
Under the current rules, a vote of no confidence can only take place once a year.
However, Ross indicated that he would not be in favour of such a rule change.
“I’m a member of the 1922 Committee, but I’m not on the executive and it’s for the executive to look at rule changes,” he said.
“There is an election I think next week or the week after and some candidates, their pitch is they would change the rules.
“I personally don’t think we should change the rules mid-way through a process, I think that’s the wrong way to do it.
“But as we saw with Theresa May, she lost a vote of the 1922 Committee and it didn’t take a rule change, she looked at the situation a few months on and she stood down herself.”