London’s first new road crossing of the Thames in decades, the Silvertown tunnel, would be an “environmental betrayal”, according to the Liberal Democrat candidate for London mayor, Siobhan Benita, who is ramping up her campaign for it to be scrapped before the May election.

Transport for London’s £1bn twin-bore tunnel is expected to take five years to build and the organisation claims it will speed up river crossings for cars and vans and reduce congestion.

Benita said no work should start on the project until the mayoral election on 7 May was out of the way and if she won she would axe it immediately. The Green party is also calling for it to be abandoned and the climate emergency group Extinction Rebellion has staged protests and is expected to carry out more in the coming months.

“Going ahead is an environmental betrayal. It will mean a disastrous increase in cars, congestion and pollution at the very moment when we should be moving in the exact opposite direction,” Benita said, as she promised to stand on the greenest ever Lib Dem platform.

Sadiq Khan, who is running for a second term with a series of new environmental targets, has said the tunnel is needed to vastly improve air quality, especially around the entrances to the nearby Blackwall tunnel, where car engines are often idle, and the number of vehicles overall is not expected to increase.

He has said new river crossings in east London are also urgently needed to ease congestion and reliability as the Blackwall tunnel was closed 700 times in 2017-18.

Benita said: “There’s nothing more important than tackling the climate crisis and for me in order to achieve that it means every single policy coming out of City Hall should be done through a green lens.

“To do that we need to start with some very basic assumptions: that we need a car and vehicle reduction target in the city, which very quickly undermines the business case for the Silvertown tunnel. Despite everything Sadiq Khan is saying, if you build a new tunnel you will get more cars.

“I wouldn’t be investing in Silvertown and I’d be reversing that decision,” she said.

The London borough of Newham, where one of the tunnel’s entrances will be built, is the most polluted borough in Britain and its fine particulate matter, or PM2.5, count exceeds World Health Organization guidelines.

Discouraging car use in London and for people to switch to walking and public transport should be the focus of any future infrastructure project, Benita said. “We shouldn’t be building any more infrastructure that leads to more cars.”

Environmental campaigners have claimed for years that the new tunnel will exacerbate traffic problems because it will attract new cars and lead to more pollution.

Instead the money could be invested in cycling and trying to make car-sharing work much more effectively in London, Benita suggested. She also wants the legislation for electric scooters to be rolled out as soon as possible, and for cargo bikes to be used for deliveries with a better coordination of goods being moved around the capital.

A London Labour spokesperson said: “Sadiq is standing up for London by cleaning up its toxic air and addressing the climate emergency. He is promising a green new deal for London and has an ambitious target for the city to be carbon neutral by 2030.

“The mayoral election on 7 May is a two-horse race between Sadiq Khan and Boris Johnson’s Tory candidate. That’s why Sadiq is asking Liberal Democrats and other progressives to lend him their vote so he can continue standing up for our city and its values.”

City Hall under Khan has backed the tunnel, which will be subject to a toll, as will the Blackwall tunnel, which is currently free to use. The extension of the ultra-low emission zone area in 2021 will also cover the Silvertown tunnel entrance and exit so drivers must have low-polluting vehicles or pay £12.50.

The crossing will also have a dedicated bus lane with up to 37 zero-emissions buses an hour in each direction.

Benita ran as an independent candidate in 2012 and came within 8,000 votes of the Lib Dem candidate, Brian Paddick.