While First Minister Nicola Sturgeon has already said she wants to hold another vote on the future of the UK “early” in the next Holyrood term, Sir Keir insisted that no responsible Prime Minister would give the go-ahead for such a ballot.
With ongoing uncertainty about the impact of the coronavirus crisis and Brexit, he said holding a second Scottish referendum would be “entirely the wrong priority”, dismissing Ms Sturgeon’s calls for another ballot as “misguided”.
The UK Labour leader said he was “determined to preserve and to renew the United Kingdom”.
He promised under Labour there would be a “radical economic and political devolution across the United Kingdom” – holding this out as an alternative to the “separatism” of the SNP and the “cavalier and chaotic approach” of the Conservatives.
Giving a virtual speech, he said that the Covid-19 pandemic had put “rocket boosters” under the case for renewed decentralisation of power from Westminster.
He announced the formation of a constitutional commission – advised by former prime minister Gordon Brown – to consider how power could further be shared out across the United Kingdom.
This new commission will “make the positive case for the UK and it will champion devolution, but beyond that it will rule nothing out,” Sir Keir added.
And while he said a “project of this scale and this urgency should be initiated by the UK Government”, he added that “in the absence of that leadership from the Prime Minister, Labour will do what is necessary in opposition”.
With Holyrood elections due to take place in May, Sir Keir also stressed the Scottish element of the commission’s work would be completed “as soon as possible”.
Here he said there was “no time to waste” as he set out how Labour would fight the next Scottish elections on the basis of opposition to a second referendum.
Ms Sturgeon has already made clear she will use the Holyrood campaign to try to win a mandate for another independence vote – with polls currently indicating a majority of Scots in favour of leaving the UK.
But Sir Keir insisted: “The last thing Scotland needs now is more years of division.”
He said: “Labour will argue passionately against another independence referendum.
“We will argue that today, we will argue that tomorrow.
“It would be entirely the wrong priority to hold another Scottish independence referendum in the teeth of the deepest recession for 300 years. While still fighting this pandemic. When there is such uncertainty about how Brexit, and coronavirus will affect us.
“And when the costs and consequences of independence are still so uncertain.”
Sir Keir added that given the “damage and division” a fresh independence referendum would cause “no responsible First Minister should contemplate it – and no responsible Prime Minister would grant it”.
He added: “There should not be another independence referendum while our economic and health outlook is so precarious – nor until there has been a proper assessment of the costs, consequences and uncertainties of separation.”
Scottish Labour leader Richard Leonard said afterwards he was “delighted” Mr Brown would advise the new commission.
Mr Leonard insisted: “Labour introduced devolution and Labour will defend it.”