An independence referendum should be blocked even if the SNP and Greens win a majority at the Holyrood election, according to the Scottish Liberal Democrats leader.

Willie Rennie said it would be legitimate for the UK Government to reject another vote in these circumstances, adding that there should not be another referendum in his lifetime.

The 51-year-old also claimed the Tories could be wiped out in Scotland at a general election and predicted his party will leapfrog Labour at Holyrood.

SNP MSP Rona Mackay said: “Self-styled liberal democrat Willie Rennie is just as contemptuous of democracy as Boris Johnson. He cannot seriously be arguing that people deserve another say on Brexit, but don’t deserve another say on independence."

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The botched handling of the Brexit process by Theresa May and Boris Johnson has led to a revival by the Liberal Democrats, who are calling for the UK to remain in the EU.

Hundreds of gains were made at the council and European elections this year and the party has also benefited from defections from Labour and the Conservatives. Jo Swinson, the leader of the Liberal Democrats at a UK level, has also presided over a poll rating surge.

Speaking to The Herald on Sunday ahead of his speech at the Lib Dem conference in Bournemouth, Rennie said his party is the only mainstream outfit that is strongly pro-EU and anti-independence.

The SNP and the Greens, both of which support independence, have been tipped to co-ordinate their manifesto commitments in a bid to secure a majority for indyref2 at the 2021 Holyrood election.

Some unionist politicians privately admit that a majority won on this basis would inevitably lead to the UK Government granting Holyrood the right to stage another referendum.

However, Rennie said: “We will still stand very firmly against another independence referendum, and we would vote against it at every single opportunity. We were promised it was a ‘once in a generation’ thing.”

Asked whether, in the event of the SNP and Greens winning a majority, it would be legitimate for the UK Government to reject another referendum, he said: “Yes, and we would argue very strongly for that at Westminster. Big bang constitutional change is chaotic, damages the economy, and divides the country.”

Put to him that this stance could be seen as anti-democratic, he replied: “No, because they [the SNP Government] made clear promises in that referendum back in 2014. It was one of the biggest democratic experiences of our lifetime. There was a massive turnout and people said no.”

Although the Brexit chaos has coincided with an increase in support for independence, Rennie denied that the case for Scotland leaving the UK is stronger.

“Brexit will prove that independence is something that should be rejected. I think the case for independence is weaker now as a result of all this.”

A recent opinion poll showed that the Lib Dems could overtake Scottish Labour, which has been divided on Brexit and indyref2, at the 2021 poll.

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Rennie predicted his party will have more MSPs than Labour in two years: “I think that’s pretty clear now. Labour are in freefall. We don’t know what they stand for anymore. They are bobbing around in the ocean. People want clarity.”

Rennie, who has led his party since 2011, is also hoping to make gains at a general election which could take place in November.

He does not put a number on the seats the Lib Dems are targeting, but says all 13 Scottish Tory MPs are at risk: "They could lose all their seats in Scotland. People are just disbelieving at the state of the Conservative party.

“Especially with Ruth [Davidson] being off the scene now, I think they will struggle to cut through.”

Asked if the Tories deserved to lose all their seats, he said: “Yes, I do, because I think the way they have handled Brexit is appalling. Why haven’t more of them followed the route of Amber Rudd [the former Tory Cabinet Minister who resigned recently]?

“Not one of them in Scotland has followed her.”

He also confirmed he would lead his party into the next Holyrood election.

Mackay added: “The fact is, the case for Scottish self-government has only been strengthened by the events of the last few years – with Westminster repeatedly ignoring the people of Scotland and dragging us toward an economic cliff-edge.

"No wonder polls show people moving towards independence and support for an early referendum – and no wonder the unionist parties are so rattled.”