John Swinney says SNP facing its biggest challenge for years | Scottish National party (SNP)

John Swinney has described July’s general election as “the biggest challenge the SNP has had for years” as he used his party’s official campaign launch to repeatedly attack Labour, which is threatening the nationalists in dozens of seats across Scotland.

Swinney, who told the rally of more than 200 activists and former MPs in Glasgow that it was a “surprise” to be leading the SNP into an election campaign, added that “voters are right to remind us never to take anything for granted”.

Keir Starmer has already travelled to Scotland twice since the election was called, pledging to make the nation “central to the mission” of a Labour government.

Swinney said that Starmer’s spending plans amounted to “austerity on steroids” and warned that “people don’t quite know what the Labour party is promising but I’m going to make sure they do”.

With polling suggesting that the SNP will pay a heavy electoral cost as it struggles to reset under its third leader in 18 months, Swinney accused Labour of doing “an awfully good impression” of the Tories, singling out recent comments by their health spokesperson, Wes Streeting, as favouring creeping privatisation of the NHS.

Swinney, who was elected leader unopposed last month after his predecessor, Humza Yousaf, resigned amid the chaos that followed his axing of the governing partnership with the Greens, told supporters on Sunday that “people want us to demonstrate the relevance of independence to their lives”.

He added: “If we don’t then we are not likely to get much of a hearing in the midst of a cost of living crisis and, to be frank, nor would we deserve to.

“So, when we talk about independence we need to demonstrate again, and again, and again, that we are talking about people’s core concerns like raising living standards and protecting the NHS.”

Attacking the Tories and Labour, he said Westminster decision-making had meant “austerity, Brexit and a cost of living crisis being imposed on Scotland”.

With polling suggesting that constitutional preferences will play a smaller role than in any election since 2015, with many independence supporters prioritising removing the Conservatives from Downing Street, the rally focused on persuading those voters to return to the SNP.

The party’s election campaign director, Stewart Hosie, told activists that it was their job to ensure that every independence supporter knows that “sitting this election out or lending your vote to another party” risked jeopardising progress on independence.

Swinney was also forced to rebuke activists who booed and heckled the BBC’s Scotland editor when he posed a question in the media Q&A.