RUTH Davidson would have to lead the campaign group to promote the UK if a second independence referendum was called, according to a former Labour MP.

Tom Harris said the Scottish Tory leader would be the obvious choice because the Conservatives beat Labour to second place at the Holyrood election.

However, Scottish Liberal Democrat leader Willie Rennie said: “We can’t leave it to Ruth Davidson.”

Read more: Ruth Davidson: My vision of Scotland's future in the union​

In the first referendum, Better Together was formed as a coalition of the three Unionist parties, with Labour figures handed most of the key roles.

Although Labour was the senior partner, the party was hammered for its alliance with the Tories.

HeraldScotland: Ruth Davidson campaigned for a Remain vote

Some senior figures believe last year’s General Election rout in Scotland, in which Labour was left with one seat, was largely down to teaming up with the Conservatives.

With First Minister Nicola Sturgeon saying a second vote is “highly likely”, attention has turned to how a successor group to Better Together would look.

Harris, who lost his Westminster seat last year and who led the pro-Brexit campaign in Scotland, is opposed to a second independence referendum.

However, he predicted his party would boycott any future tie-up with the Tories: “The Labour Party will refuse to get into another Better Together-type coalition - they will be wrong in doing so, but it is not the first wrong decision the Labour Party has made since 2014.

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“It should be about saving the Union. It shouldn’t be about party advantage.”

He also said Davidson would front the group: “Ruth is now the leader of the opposition in Scotland. She would have to lead that campaign and that would be difficult for the Labour Party.

HeraldScotland: Tom Harris is the Director of Scottish Vote Leave, the officially designated “Leave” organisation in the EU referendum campaign.

“But they are going to have to accept the judgment of the voters. It is not the Tory party that put Labour in this position. It’s the Labour Party that put the Labour Party in this position.”

Rennie, whose party also played a key role in Better Together, dismissed the idea of Davidson leading any campaign: “It’s really important there’s a progressive, centre-left moderate, pro-UK voice that is very strong, distinct and clear in this campaign. That’s why we can’t leave it to Ruth Davidson.”

He added that Sturgeon would “love” Davidson to be the leader of any official pro-UK campaign group.

Rennie also accused the Tories of harming the Union: “The Conservatives bungled the post-referendum victory by turning it into English nationalism and he [David Cameron] followed that through into the General Election. He cocked up Brexit, which has put more strains on the Union.”

A senior Unionist source said the structure of any future campaign group was a “worry”.

He said: “The problem is we have no-one on Nicola’s level. Gordon Brown and Alistair Darling have moved on and there is no-one of that stature.”

Read more: Nicola Sturgeon: Independence 'transcends' Brexit, oil and the economy​

A Scottish Conservative spokesman said: “There should be no need to talk about indyref 2 because the first referendum was clear - the people of Scotland wanted to stay part of the UK two years ago and still do now.”

A Scottish Labour spokesperson said: “It is clear from recent polling that the people of Scotland do not want a second independence referendum. Labour's focus will be on holding the SNP to account and using the powers of the parliament to support strong public services and economic growth.

“The Tories want to rerun the arguments of the past because they have no ideas for our nation's future.”