Sir Keir boasted that his party “blew the doors off” in the Scottish contest, marking a “big step in the right direction” on his path to Number 10.
Labour’s higher-than-expected 20.4 percentage point swing from the SNP led analysts to believe that it could return to being the largest party north of the border and open the door to Downing Street if it is replicated at the next national poll.
But despite the win in the Rutherglen and Hamilton West by-election, the Labour leader faced fresh ridicule over his policy “flip-flops” – most recently on HS2. Sir Keir has also been scaling back or abandoning a range of policies, including dropping a pledge to abolish tuition fees and delaying the full implementation of a £28 billion green fund. He has also been muted on protecting the Triple Lock on pensions.
Even Senior Labour figures are concerned that the party’s chances in next year’s national vote could be scuppered by Sir Keir.
Craig Tracey, vice-chairman of the Conservative Party, said: “We know from recent experience that by elections don’t really translate to the results in future general elections.
“What is evident though is that there hasn’t been any major shift to Labour in these elections, where the results have been based on low turnouts by Conservative voters rather than an increase in numbers coming over to support Labour.
“This was also true of last nights Rutherglen & Hamilton by election where they won with fewer votes than they lost to the SNP with in 2019.
“Starmers message, whatever that may be, isn’t cutting through and voters are not being fooled, meaning there is still everything to play for at the next General Election.”
Fellow Tory Mark Jenkinson said: “He might be boasting today, but tomorrow he’ll have changed his mind again anyway.”
He said Labour’s by-election result was largely due to a collapse in turnout and wouldn’t be replicated on the national stage.
“We have seen a collapse in turnout in recent by-elections, such that they can’t be taken as anything other than what they are – isolated wins for a party that has just quietly tried to drop from its website every policy it’s launched over the last two and a half years,” he added.
Speaking at a victory rally alongside winning Labour candidate Michael Shanks, Sir Keir said: “They said that we couldn’t change the Labour Party and we did it.
“They said that we couldn’t win in the south of England and the north of England, and we did it. They said ‘you’ll never beat the SNP in Scotland’ and Rutherglen, you did it. You blew the doors off!”
Polls suggest that Labour has around a 17-point lead over the Tories.
Labour candidate Mr Shanks secured 17,845 votes, well ahead of the 8,399 votes returned for his closest rival, the SNP’s Katy Loudon.
With Mr Shanks, Labour has two MPs in Scotland – far off its peak of more than 40 in 2010 before its representation collapsed after the 2014 independence referendum.
But the result is being seen as a change in the Scottish political weather and a path to a Labour majority in the House of Commons.
It confirms Labour poses “a serious challenge” to the SNP’s primacy at Westminster and has a momentum comparable with the run-up to the party’s 1997 landslide, according to polling guru Professor Sir John Curtice.
“That potentially has implications for the overall outcome in the general election because if that were to happen, they would find it easier to get an overall majority,” he said.
Sir Keir is under huge pressure to set out a vision for Labour when the party gathers for its autumn conference in Liverpool this weekend.
It comes as key figures from Tony Blair’s 1997 campaign have warned Sir Keir’s caution risks damaging his chance of winning the next election.
Alastair Campbell, Blair’s former director of communications, told the Guardian: “You want policy, and you want your policies to be known about, and you want to be able to defend them and to argue them.
“I sometimes worry that the Labour party today doesn’t. I don’t often get that sense of that sort of relentless, restless, obsessive attention to detail focusing on everything that you need to focus on “[In 1996 and 1997] we were not happy if we were not making the news and we weren’t making the weather and we weren’t actually being attacked. Because sometimes being attacked is the only way that you can get out there and make your case.”
Lord Mandelson, Labour’s campaign director for the 1997 election, and a close confidant of Sir Keir, said: “Do I think the Labour party under Keir Starmer is ready for government? Well actually, I don’t. But then the election isn’t going to take place tomorrow or the day after. They have another year.”