However, in the wake of a series of rows – including about the influence of his chief of staff Sue Gray – his approval rating has now slumped to -26%. That is a staggering drop of 45 points.

At the same time, Mr Sunak’s approval rating has risen by five points in the past fortnight to -25%.

However Sir Keir is still ranked ahead of Mr Sunak and the best prime minister, beating him by 28% to 18%.

Part of Sir Keir’s pitch to voters at the General Election was that the Conservatives had left the country worse off and should be replaced.

But despite repeatedly claiming that the Tories left behind a £22billion black hole and that the incoming administration must now “fix the foundations”, Opinium said: “The public is divided on whether the new Government is on par with its predecessor, with 30% thinking Keir Starmer’s Labour Government is better than the last Conservative government under Rishi Sunak, and 34% thinking it is worse.”

Only 27% of those questioned thought the Government’s first months in office had been a success, while the poll also showed Labour is not seen as a united party. In total, 32% said it was united and 37% not.

The bombshell findings are likely to lead to questions about Labour’s controversial decisions since it came into office, heaping more pressure on Sir Keir at his first party conference as PM in the next week.

There has been widespread anger at the decision to cut winter fuel payments, with many campaigners arguing it will harm poorer pensioners this winter.

It is highly unusual for a government to become so unpopular so quickly. Sir Tony Blair’s approval rating famously hit 93% in one poll the September after his 1997 General Election win, following his “people’s princess” speech when Princess Diana died.

Sir Tony lasted 1,104 days before Labour’s approval rating dropped below 30%. Another analysis published earlier this week showed Sir Keir’s administration has hit that level in just 70 days.

Even Gordon Brown took 1,082 days to take Labour to below 30%, despite being roundly slated for not calling a snap election shortly after he became PM in 2007.