Sir Tony Blair has accused Sir Keir Starmer's government of having no "coherent plan" for the country and introducing policies that have held back business.
In a highly critical essay, the former Labour PM singled out measures including new workers' rights laws, the phasing out of the British oil and gas industry and the above-inflation uplift to the minimum wage.
However, he warned that whether there was a change of leader was "irrelevant if it doesn't start with a policy debate".
Downing Street declined to comment on the essay before it was published, but said Sir Keir was "fully focussed on delivering change for working people".
A spokeswoman pointed to measures aimed at easing the cost of living, with the economy growing before the Iran conflict broke out, as well as falling NHS waiting lists, migration and serious violent crime.
She added: "But there's obviously much more to do, and we are getting on with that job."
The essay of more than 5,600 words is Sir Tony's first in-depth critique of Sir Keir's government.
It comes as the prime minister is under severe pressure, following a disastrous set of election results earlier this month and five ministerial resignations, with a leadership challenge widely expected.
Sir Tony said the government's "principal problem" was not "Keir's personality" or "a failure to communicate 'our achievements'", which have often been pointed to as weaknesses of the PM.
He wrote: "It is because we don't have a worked-out coherent plan for the country in a fast-changing world and are in the wrong political position from which we can devise one and win a second term".
However, he added: "Trying to force the prime minister out before we know what policy direction we're bringing in, is not a serious way of conducting ourselves."
While Sir Tony said he agreed with some of the government's policies, including investment in infrastructure, reform of the planning system and reducing trade friction with Europe, the former PM said other commitments were "unwise to proceed with" given the current economic circumstances.
He pointed to new workers' rights laws, which have faced criticism from some business groups who argue they will discourage companies from hiring and hit economic growth.
Sir Tony also criticised the decision to increase National Insurance for employers, which he said had undermined business confidence.
"Then, in the last Budget, it appeared as if we were increasing tax to pay for additional welfare spending, when the public already thinks welfare bills are too high," he said.
"Taken together, these measures have given headwinds not tailwinds to British business despite the macroeconomic gains for which the chancellor is rightly praised."
He called on the government to try and limit the effect of these changes and remove parts of the net-zero agenda "which prioritise clean energy over cheaper energy".
On the UK's relationship with the European Union, Sir Tony said "Britain has lost from Brexit" and "at some point it is ripe to enter a debate about 'going back'."
Wes Streeting, who resigned as health secretary earlier this month in protest at Sir Keir's leadership and has confirmed he would stand in any contest, recently argued that the UK should rejoin the EU one day.
However, Sir Tony said: "Just as Brexit was never the answer to Britain's challenges back in 2016, reversing it isn't the answer to the country's far worse situation in 2026."
He added: "If we want to go back into some sort of structured relationship with Europe, we can only do so from a position of economic strength."
Setting out his own vision for change, Sir Tony said Labour must become the "Radical Centre", putting "policy first and politics last".
He said this could include removing obstacles to business growth, welfare reform, action to tackle illegal immigration and a harnessing of artificial intelligence.
Chris Curtis, Labour MP for Milton Keynes North, told BBC Radio 4's The World Tonight that he found Sir Tony's essay "quite refreshing to read" as it sought to "actually start to grapple with the very big issues that we're facing".
But Labour MP Rachael Maskell said she did not believe Sir Tony's analysis was correct, telling the same programme: "Tony Blair won an election nearly three decades ago and it seems he's continuing the argument from back then rather than looking at the situation today."
The York Central MP also described the timing of the intervention as "incredibly unhelpful" due to three parliamentary by-elections next month.
This includes the contest in Makerfield, which will be crucial for the future direction of the Labour Party.
Labour is facing a challenge from Reform UK, which performed strongly in the area in May's council elections.
The party's candidate is Greater Manchester mayor Andy Burnham, who is seeking to return to Westminster and is expected to challenge Sir Keir for the Labour leadership if he wins.
You can see a full list of the candidates standing in the Makerfield by-election here.
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