John Curtice: Result of crunch Holyrood by-election 'in lap of the gods'

Sir John, a professor of politics at the University of Strathclyde, said national polls suggest a close contest between the SNP and Labour, adding: “This one’s in the lap of the gods, I think.”

Holyrood’s Presiding officer Alison Johnstone has informed the chief executive of South Lanarkshire Council that the poll for the Hamilton, Larkhall and Stonehouse seat will be held on 5 June.

It follows the death last month of Christina McKelvie, who had represented the constituency since 2011. The 57-year-old was on medical leave from her role as drugs minister and had been receiving treatment for secondary breast cancer.

The by-election will be the first for Holyrood since 2019, and is already being seen as a key test for both Labour and the SNP ahead of next year’s Scottish Parliament election.

Sir John said great care needed to be taken about suggesting its outcome will necessarily have any bearing on the Holyrood election, insisting “we live in a highly uncertain world and who knows where we will be in 12 months’ time”.

However, he said if Scottish Labour does not win, “it will be confirmation of the fact that they should already be very seriously concerned”.

He added: “Labour could win the by-election while doing much less well than they did in the general election, and doing no better than they did in [the last Holyrood election in] May 2021.”

Sir John said opinion polls still show the SNP about 13 points down on where the party was in May 2021.

“If, and it’s a big if, the national polls are roughly right and if the by-election swings in line with them - and of course by-elections always have their own idiosyncrasies - basically we’re looking at something pretty close to a dead heat, and therefore constituency factors will matter,” he told The Scotsman.

“And in particular, do the Greens stand, do Alba stand, and therefore perhaps split the nationalist vote? How much damage do Reform do to Labour? To what extent can Labour squeeze the Tory vote in the way that they were clearly able to do so last summer, but maybe now might prove to be more difficult? This one’s in the lap of the gods, I think.”

He added: “I don’t think you can make a prediction. I think that’s the point. It looks like a 50/50 shout. But the fact that it’s a 50/50 shout rather than [it being clear] the SNP are going to hang on to it tells you we are not in the same world as in May 2021.

“But equally, it also tells you we are not in the same world as [the general election in] July 2024. We are in between those two worlds, where basically the SNP is a little bit stronger than they were in July, but they are much weaker than they were in May 2021, and the Labour Party are much weaker than they were last July, but maybe doing just about as well as they did in May 2021.”

Nigel Farage’s Reform UK is splitting the unionist vote, Sir John said, and its rise has “helped to transform the SNP’s electoral prospects next May”.

The SNP has already announced South Lanarkshire councillor Katy Loudon will seek to keep the constituency in the party’s hands.

She was previously the candidate in the Rutherglen and Hamilton West Westminster seat at both the election last summer and the by-election in 2023.