London mayoral candidate Zac Goldsmith has backed plans to convert the A3 at Tolworth into an underground tunnel, while rival Sadiq Khan promised to pursue the proposal only if it offers “good value for money”.

Plans first surfaced to take the A3 – one of London’s busiest commuter routes – underground when Boris Johnson visited Boston last year and suggested the move would free up land for development and tackle pollution.

With Mr Johnson’s replacement due to be elected in May, Conservative candidate Mr Goldsmith said he is “committed to finding a cost-effective way to deliver it”.

He said: “I am particularly concerned about air pollution levels around the Tolworth area, and I’m pleased that Transport for London (TfL) is looking at building a tunnel in Tolworth to help ease congestion.”

The plans are based on the Big Dig project in Boston, where a major interstate highway was sunk to make a 3.5-mile underground tunnel.

Concept drawings show the proposed tunnel in Tolworth would extend from the roundabout at Tolworth Tower to beyond Princes Avenue, including the site recently sold by Tesco to property developer Meyer Bergman.

Labour’s mayoral candidate Sadiq Khan said: “Air pollution is a blight on the people of London and something that must be addressed. If a proposal doesn’t give good value for money then it won’t happen.”

TfL said in February 2015 it would draw up more detailed plans with Kingston Council and report back to the mayor’s office in May of that year, including with a funding plan.

A spokesman for the Mayor of London could not confirm when the proposals would be finalised.

Kingston and Surbiton MP James Berry said: “In my opinion the plans aren’t ambitious enough.

“I would like to see the whole of the A3 tunnelled through the borough.”