DEPLETING support for the SNP in opinion polls is now being reflected in council by-elections, showing the party’s dominance at Westminster is under threat at the next General Election, Professor John Curtice has said.

The SNP lost a second by-election in as many months on Friday as Labour gained a seat in the East Kilbride West ward in South Lanarkshire. Anas Sarwar’s party also took a seat in Bellshill off the SNP in June.

But polling expert Curtice told The National what is significant is not the loss of the seats for the SNP, but the fall in their vote share, which provides more solid evidence – beyond recent opinion polls – that they are not commanding the support they once did from Yes voters.

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In East Kilbride West, the SNP first preference vote share was down 8.2% while in Bellshill it was even more significant, dropping 13%.

This compared to a surge in support for Labour in both areas. In East Kilbride West they saw first preference backing increase by 14%, while in Bellshill it went up 13%.

Curtice said: “Who gains and loses a seat in a by-election is a poor measure of performance. You need to look at the change in the first-preference vote because parties often lose seats because they picked up the third seat in a ward, therefore it’s impossible for them to defend it in a by-election.

“In this case what is clear is the SNP vote is down by 8% compared with the first preference vote last time, that’s the crucial thing to note.

“We know the opinion polls have said support for the SNP is down and it’s useful to note that is confirmed by these by-elections.”

Asked whether the results give any indication of the SNP potentially struggling at the next General Election, Curtice said: “We know the SNP’s dominance of Scottish seats at Westminster is under threat.

“It will take very small shifts in either direction to either see the SNP still likely to pick up most of the seats or for the Labour party to pull ahead. It’s all very much on a knife edge.

“There is a real fight and the SNP have a challenge on their hands to maintain their position at Westminster. There’s no doubt about that.”

Labour’s recent success though was downplayed by Curtice as he insisted it had nothing to do with a downturn in fortunes for the SNP.

He added: “The principle foundations of the rise in Labour support is nothing to do with the SNP and everything to do with Liz Truss and partygate.

“The party north of the Border has been profiting from the same forces that the party south of the Border has. More recently, you can add in the fact support for the SNP has fallen.”

“Labour are the bridge party, they are the one Unionist party that do have some ability to pick up support of people in favour of independence.

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“What is clear is support for independence is still running at 47/48%, but the SNP no longer commands the support of Yes voters in a way that it did a couple of years ago.

“That’s the most immediate problem the party have to solve.”

Both the Tories and Labour saw their percentage vote share increase in the three-member East Kilbride West ward, with the Tories vote increasing by 6.4%, according to Ballot Box Scotland.

Murdo Fraser and Jamie Halcro Johnston were spotted congratulating Labour on Friday as the former noted a 70% Unionist vote in a hark back to the Better Together days of the independence referendum.