The former prime minister, who dramatically resigned as an MP last month, pointed to divisions within Labour's ranks over the expansion of the scheme, saying members of the party hierarchy "have begun to ditch Khan".

Earlier this week, Sky News reported that the Labour candidate in Uxbridge and South Ruislip - Mr Johnson's former constituency - had called for a delay to the zone's extension due to the rising cost of living, despite previously defending the policy.

The Conservatives - rocked by the controversial nature of Mr Johnson's resignation - hope to win support in Uxbridge by capitalising on residents' opposition to the mayor's plan to expand the zone at the end of next month.

ULEZ, which was launched in April 2019, currently covers central London and the area up to, but not including, the North and South Circular Roads.

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Mr Khan plans to expand the zone up to the capital's borders with Buckinghamshire, Essex, Hertfordshire, Kent and Surrey, a move that would bring around five million more Londoners into the scheme.

Mr Johnson cited the case of an 82-year-old man in Epsom who would be penalised by the daily £12.50 charge to go to his church in Orpington, Kent, because his old car does not comply with the zone's emissions standards.

" A tax must be fair; it must be justified; and it must be timely - and this tax fails on all counts," Mr Johnson.

"This ULEZ £12.50 tax on driving into London - and driving around outer London - is blatantly unfair in that it penalises such elderly motorists as the churchgoer in Epsom who are likely to have older  vehicles."

He added: "It is the sheer bone-headed cruelty of Sadiq Khan's scheme - rushed in with only nine months' consultation - that is causing such indignation; and it is that indignation that is fomenting the panic in Labour ranks.

"For months the Labour stooges have played along with his claims. They have supported the ULEZ tax. Sir Keir Starmer has backed it fervently, in so far as he is capable of being fervent about anything."

However, it was in fact Mr Johnson who conceived ULEZ as an idea when he was London mayor from 2008 until 2016.

The former prime described ULEZ as "an essential measure to help improve air quality in our city, protect the health of Londoners, and lengthen our lead as the greatest city on earth" when he announced it in 2015.

Mr Johnson tacitly conceded ULEZ had been the idea of the "last Conservative mayor", but added: "It never occurred to us to impose the ULEZ on the whole city, because - as Khan has now discovered - there are vast tracts of London where it is simply the wrong tool."

Where will the expanded ULEZ zone cover?

A source close to the mayor told Sky News: "This is utter nonsense from the disgraced former mayor.

"It was Boris Johnson who first announced the introduction of the ULEZ to tackle air pollution, and around 4,000 Londoners still die prematurely every year as a result of toxic air.

"Boris Johnson also made the woeful deal with government to remove TfL's entire operational grant at a cost of nearly £1bn a year."

A handful of Labour politicians have begun to speak out against the scheme's expansion, which is due to take place at the end of next month.

At a hustings chaired by Sky News' political correspondent Rob Powell earlier this week, Labour's Uxbridge candidate Danny Beales said it was "not the right time" to expand the scheme due to residents' concerns over the rising cost of living.

His concerns have been echoed by Labour MP Siobhain McDonagh, who said Mr Khan's policy would "make lives harder".

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Sir Keir has also struggled to state his position on the ULEZ expansion, telling LBC that the mayor has "no choice" but to go ahead with his plans, while also defending Mr Beales's decision to speak out against them.

Addressing a listener question on LBC, Sir Keir said: "I accept that the mayor has no choice but to go ahead because of the legal obligation on him.

"I think Danny is right to stick up for his constituents.

"I understand the pain it is going to inflict."

There are fears within Labour that the policy could be damaging to the party in the Uxbridge by-election, with one senior Labour MP telling Sky News: "We could be in a situation where we win the others [by-elections] but we lose Uxbridge because of the ULEZ."

The Uxbridge by-election will take place on 20 July, alongside contests in Selby and Ainsty and Somerton and Frome, where the Conservatives are defending majorities of around 20,000 votes.