Boris Johnson has attacked Sadiq Khan for using “flimsy” arguments to block sending cars to Ukraine that would otherwise be scrapped under Ulez.

The former prime minister, who was Mr Khan’s predecessor as mayor of London, told The Telegraph he believed Londoners would want to play their part in helping Ukraine fight Vladimir Putin’s regime by sending their cars to the front line.

Mr Khan has claimed Ulez non-compliant 4x4s cannot be used by soldiers in Ukraine because they do not meet the “legal threshold” that requires the scrappage scheme to benefit Londoners from “an economic, social and environmental perspective”.

However, Mr Johnson told The Telegraph: “It strikes me as being petty and ridiculous. It’s rubbish.

“The Ukrainians are more than capable of solving the quality of air problems themselves, what they need is help now to defeat a threat that is infinitely more dangerous and that is President Putin. Khan’s actions seem to be extraordinary.”

In December, Mr Khan wrote to Vitali Klitschko, the mayor of Kyiv, saying he would not permit vehicles being prepared for the Ulez scrappage scheme to be sent to Ukraine.

Mr Johnson, who served as London mayor between 2008 and 2016, questioned Mr Khan’s argument under the Greater London Authority Act 1999.

“It is small-minded of Khan,” Mr Johnson said. “His legal reasons for doing so are very flimsy. It obviously benefits Londoners, in a sense that Londoners deeply value and cherish the idea of Ukrainian freedom.

“London’s success depends on its freedom and democracy. It’s vital for London that we defend these principles everywhere.”

Since Russia’s invasion of Ukraine, numerous volunteer organisations have been driving donated 4x4s and larger vehicles for soldiers to use on Ukraine’s front line.

Once inside the country, the vehicles are painted with camouflage, fitted with machine-gun or rocket launchers, and used for everything from fighting to retrieving casualties.

Gove seeks to overrule mayor’s decision

Mr Johnson added that he was “in favour” of Michael Gove’s recent letter to Mr Khan, in which the Communities Secretary set out that he would seek to overrule the mayor’s decision using the power granted to him in his role.

“Ukraine will confirm that the British people share a particular affinity for the Ukrainian cause and a particularly strong moral belief, as adamant as any population in the world, in [their defence],” Mr Johnson said, adding: “People in Britain want to see a free Ukraine and I’m sure they will want to send motor vehicles and anything they can to support Ukraine.”

However, Mr Johnson, who was prime minister when Putin invaded Ukraine in 2022 and who struck up a close relationship with Ukrainian president Volodymyr Zelensky, said the vehicles were more of a “gesture”, compared with the air power the warzone really required.

“We have to accept that this is a very small thing to do but it’s something we could do, and frankly we should be doing more,” Mr Johnson added.

“Although this is a good thing, it’s frankly trivial by compassion. We need to be giving Ukrainians air cover and de-mining capability and long-range artillery fire that they need to expel the Russians and Putin’s army from their country.”

A spokesman for the Mayor of London said: “Sadiq is calling on the Government to temporarily alter the national regulations for the certificate of destruction, which is required as proof that a vehicle has been permanently scrapped, to enable the export of suitable vehicles to Ukraine via a registered charity or national scheme.

“Officials are already in discussions and we are optimistic the Government will listen to our pleas to enable Londoners to receive money for taking polluting vehicles off London’s streets while helping the people of Ukraine.”