Green mayoral candidate Sian Berry will launch her campaign today by accusing Sadiq Khan of being disingenuous about his environmental record.

The Green party co-leader is expected to criticise the mayor of London for his support of the £1bn Silvertown Tunnel and airport expansions, while labelling his environmental policies as “lip service” to “green issues”.

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She is expected to say: “Sadiq Khan has failed to be green when it most matters.

“The current mayor backs airport expansion at Gatwick and City Airports and will spend £1 billion on the toxic urban motorway tunnel at Silvertown.

“This mayor is asking you to trust him to tackle climate chaos – but this same mayor ordered climate protesters to ‘get back to business as usual’.”

Khan has been trumpeting his green credentials over the past weeks, yesterday announcing a £50m green energy fund in a bid to turn London carbon neutral by 2030.

He followed the announcement by asking Green supporters to “lend him” their vote as “Tory candidate [Shaun Bailey] and I are the only candidates who can win this election”.

Berry, who also ran for mayor in 2016, hit back by saying the mayor must not understand the voting system for mayor of London elections.

The electoral system sees voters preferencing candidates from one downward, with the two candidates with the most first preference voters progressing to the second round.

Preferences from eliminated candidates are then awarded to the final two candidates to determine the winner.

Berry said that this meant that it was not a two-horse race at all as it would be under the first past the post voting method.

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She said: “Is it inconceivable that the current Mayor doesn’t understand the voting system for his own job?”

Berry has already pledged that if she becomes mayor she would get rid of Transport for London (TfL) zones and introduce a “flat fare” scheme, expand the Ultra Low Emissions Zone (Ulez) for the whole of London and oppose any new road river crossings.