'Operation Branchform has led to voters losing trust in SNP'

Ms Cameron, who is the MP for East Kilbride, Strathaven and Lesmahagow, said: "I think there are issues of trust and competence. You've got people being arrested and questioned by the police. Concerns are being raised. I am not going to comment on the outcome but it is an issue the public are aware of.

"You have a camper van arriving, that I didn't know existed, which was supposedly a campaign vehicle."

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Ms Cameron is the latest politician to make a link between the ongoing Police Scotland investigation and the fall in the SNP's popularity with SNP figures also raising concerns over its impact.

Polls over the last year suggest the party has lost support among voters since Ms Sturgeon announced her resignation in February last year. In October last yer the party heavily lost the Rutherglen and Hamilton West by election to Labour.

Scottish Conservative MP Lisa Cameron.  Photo Colin Mearns

Ms Cameron said: "When you tap doors, people are asking 'how do we beat the SNP in this seat'. Some of them are former SNP voters, some people say they won't vote.

"I think even within SNP voters for those who really want independence that they have not done enough on that. They maybe have focussed too much on progressive idealistic type politics.

"So I feel Scotland is turning a page on nationalism. A lot of people are moving back to where they started before 2014. They are moving back to left or right of centre and I think that is what will happen at the election."

Former first minister Nicola Sturgeon, her husband Peter Murrell, the SNP's former chief executive, and Colin Beattie, the SNP's former national treasurer, were arrested in the ongoing Operation Branchform into their party's finances last year. All three were later released without charge pending further inquiries. Ms Sturgeon later told journalists she was "innocent of any wrongdoing."

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In her first major interview, since months on from crossing the floor of the House of Commons, she also denied the Prime Minister promised to give her a seat in the Lords if she defected from the SNP to the Conservatives. At the time of her switch there was some speculation that a deal over a peerage had been done.

But she said no such offer had been made.

"I didn’t discuss anything about the Lords with the PM and I was not offered to go House of Lords to move party," Ms Cameron told The Herald.

"That was not part of the discussion I had with the Prime Minister and it is not something I would expect at all."

She said the 40-minute meeting was arranged by a UK Conservative MP.

"The meeting with the Prime Minister took place in Parliament. It was to have a discussion about where I was at and what I might be thinking. At that point I was thinking of leaving the parliament," she said.

"The Prime Minister said I had done a lot of good work on disability issues. I felt he was very empathic. He was listening. He was interested in my family which no one had been particularly interested in before.

"He asked how my girls were. He has two girls of similar ages to mine. They are all into horse riding. There was just a kind of connection.

"The meeting lasted about 40 minutes. I discussed some of the issues I had [in the SNP]. I discussed that my husband was a councillor and a veteran and that veterans issues are important to me. We are basically an armed forces family.

"I talked about being a Christian in parliament. I spoke about what I hoped to do in parliament which was really not to focus on constitutional issues which divide families and divide the country."

Ms Cameron also went on to say voters in Scotland were concerned over the provision of education, about NHS waiting times and the delays to two ferries being built at Ferguson's shipyard in Port Glasgow.

Polls suggest that the SNP could lose some of their seats in Glasgow and the central Belt at the general election expected next year while also facing a challenge in rural Scotland and the Highlands from the Conservatives.

But they still mostly point to the SNP winning the general election in Scotland with Labour coming second and the Conservatives third.

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She will step down as an MP at the general election and defended her decision not to resign as an MP after moving to the Conservatives - as the SNP called for.

"I have never been busier as an MP. I have a fifth more emails than I’ve ever had. People have not stopped coming to me as a Conservative MP," she said.

“Business have been in touch who weren’t in touch before. I’ve had constituents who are Conservative who got in touch who didn’t before, and I still hear from and represent my SNP supporting constituents.”

A survey published by Ipsos Scotland on Friday found fifty-six per cent of those questioned said the Scottish Government had performed badly in improving the state of NHS, and 49 per cent believed ministers had performed poorly in improving living standards for those on low incomes.

Forty-eight per cent thought the Scottish Government had performed badly on education and 47 per cent said it had been poor at managing the economy.

An SNP spokesperson said: “The Tories have trashed the economy and dragged Scotland out of the European Union against it's will.

"In contrast, the SNP is doing everything it can to mitigate Westminster's economic mismanagement - including lifting 100,000 children out of poverty and investing record sums of money into our National Health Service.

"At the next election, it is only the SNP who can make Scotland Tory free, stand up for Scotland's values and defend's Scotland's interests at every possible opportunity."