THE SNP's tactic of putting a second referendum at the heart of its campaign has been called into question by a senior party figure as support for independence fell to its lowest poll rating in five years.

Brexit has also been cited as a reason for the fall in support by a top academic, who suggested some voters may see the chaos over leaving the EU as an indication of how difficult breaking away from the union could be.

The SNP, which still has a high popularity rating among voters, is predicted to win the majority of Scottish seats in Thursday's General Election, but one senior party source said their strategy "has probably meant the Tories are going to do better than we hoped".

A YouGov poll of 1002 Scots published yesterday showed that support for leaving the union was 44 per cent – 1 percentage point lower than that achieved during the 2014 referendum.

The previous poll by the same firm in September showed 49% of respondents said they would vote in favour of independence while 51% would vote against.

A senior SNP source told The Herald on Sunday the lack of dynamism in the party's campaign messaging may have cost them support for leaving the union.

He said: "I think the campaign hasn’t quite understood what the central message is – Stop Brexit in some places, People’s Vote in others, Stop Boris, and then Indyref in others.

"I think the lack of a coherent message, and the leadership being too risk-averse, has probably meant the Tories are going to do better than we hoped."

Nicola Sturgeon has not addressed the poll results directly but yesterday joined Westminster candidate Fergus Mutch in Aberdeen where she urged voters to unite around the SNP to stop Boris Johnson re-entering Number 10, and to prevent Brexit.

One academic said the SNP had "no choice" but to campaign for a second referendum, and the decline in support was likely to be the "Brexit effect" rather than a drop in popularity for the SNP itself.

Rob Johns, professor of politics at the University of Essex, said: "The SNP don't have that much choice on what they focus on. They have an enthusiastic base, part of which is much more enthusiastic than they perceive the leadership to be.

"If the leadership downplayed independence they would have a lot of people asking them 'What the hell?' effectively.

"The SNP tends to do best when voters think there isn't going to be a majority, so they think that a lot of SNP MPs could really make a difference for Scotland. Realistically when you look at what voters believe this election is going to be, most of them think it will be a majority Tory Government which will do what it wants.

"As a result of that perception, the SNP doesn't really have that angle to go at, so it kind of had to campaign for independence.

"As to what effect it's had, firstly it is only one poll. Secondly this is the biggest switch we've seen for ages, but it's not a huge switch in absolute terms – it shows how kind of engrained these things are. It is not as if support has collapsed.

"It's hard for us to say what has caused it but my best guess is that it's less about the way the SNP has campaigned and more about the Brexit effect highlighting the difficulties that independence could bring.

"It's hard to say there would be any sort of smooth transition to independence.

"It doesn't mean that people don't want independence, but it has amplified the reasons why people might be wary of voting for it now."

Johns added that since devolution, domestic issues which are the responsibility of the Scottish Government are still brought up by opposition parties during general elections as a way of "having a go" at the SNP's record.

He said: "Post-devolution, during general elections it is noticeable when they are not talking about independence. The opposition wants to talk about other issues like health and education, but strictly speaking that isn't up for grabs in this election.

"It's a way of having a go at the SNP.

"The SNP doesn't want to feel vulnerable on their domestic record, it doesn't really want to talk so much about domestic performance."

Annie Wells, the Conservative MSP, yesterday criticised the Nationalists on their performance in devolved issues including health and education, while Richard Leonard, the Scottish Labour leader also hit out at health inequalities under the SNP.

Wells said: "People are sick of referendums, whether it’s independence or Brexit.

“Both these need to be put to bed so Scotland can move on, and the Scottish Conservatives seem to be the only ones wanting to do that.

“People are clearly getting sick of Nicola Sturgeon’s endless lecturing on the break-up of the UK. This poll comes at the end of a week where the SNP’s domestic record has been brutally exposed.

“Voters are increasingly seeing that a decade of independence distraction has heavily compromised health, education and other matters the SNP is responsible for.”

Yesterday Scottish Labour launched a plan to tackle "deepening health inequalities" across the country, with Leonard saying people were growing tired of "constitutional bickering"

He said: "It is clear that the people of Scotland are weary of constitutional bickering and are looking actively for a programme that will tackle the blight of child poverty, end food bank Britain and give working people a say in the running of the economy.

“That is what Labour is proposing in this election. Labour provide a radical alternative to the status quo and will bring about real change for Scotland and Britain.

“ Labour will provide £100bn in additional investment for Scotland over two terms, eclipsing the SNP’s case for the construction of a separate Scottish state, and place the people of Scotland in the driving seat of the economy."

Scottish Liberal Democrats leader Willie Rennie, meanwhile, said the decline in the support for independence was partly down to Brexit fatigue.

He said: "Support for independence is down for a multitude of reasons including the realisation that independence would be just like Brexit.

"People are exhausted with all the constitutional chaos of Brexit and independence we have endured for years and want to focus on matters that improve people’s lives like mental health, childcare and the climate.

"People certainly don’t want another divisive independence referendum within months as the SNP want. That’s why ever more people are backing the Liberal Democrats to stop independence as well as stopping Brexit.”

An SNP spokeswoman said: "This crucial election is about locking Boris Johnson out of Downing Street, escaping Brexit and securing Scotland's right to choose.

"All other recent polls have shown that the issue is a virtual dead heat and that support for independence is rising, including a rise in support for having an independence referendum.

"A vote for the SNP is a vote to put Scotland's future in Scotland's hands – not Boris Johnson's."