Scotland faces becoming a one party state under the Scottish National Party as Labour struggles to fight Nicola Sturgeon's party north of the border, David Cameron has warned. The UK prime minister claimed the Scottish Conservatives were the only party capable of challenging the nationalists' dominance in the polls.

Cameron made the pitch to Scottish voters at the Scottish Conservative annual conference in Edinburgh on Friday 4 March, just two months before the Holyrood elections in May.

"We are now the only party that can properly challenge the SNP," the Conservative leader declared. "They've been in power for nine years, they are the establishment and, with Labour's collapse, Scotland is in danger of becoming a one-party-state."

He added: "The first thing we need to tell voters is about the UK. We always said we were the party of the Union. But now it's clear we're the only party of the Union. For the SNP, it's still independence or nothing despite us settling the matter for a generation.

"And now we've got Labour – and the Lib Dems – saying their politicians can campaign either way if a second referendum should ever take place, effectively abandoning their support for the Union.

"For Scotland, this is huge. It leaves just one party – the Conservatives and one person – Ruth Davidson – to speak up for the two million people who voted No in that [2014] referendum."

Cameron's keynote speech comes after Labour were reduced from 41 to one seat in Scotland after the 2015 general election.

The Liberal Democrats also witnessed a near wipe-out, with just Alistair Carmichael surviving. The SNP, however, romped to victory and gained 50 constituencies at the May ballot, bringing their total number of MPs to 56.

Former minister Jim Murphy resigned as Scottish Labour leader in reaction to the result and MSP Kezia Dugdale has been at the top of the party since August 2015. But despite the reshuffle and Jeremy Corbyn's surprise election as Labour leader, Scottish Labour are far behind the SNP in the opinion polls.

The latest survey from Survation for The Daily Mail, of more 1,000 people between 25 and 29 February, put the SNP on 54%, Scottish Labour on 21% and the Scottish Conservatives on 16%.