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Boris Johnson today gave the green light to a scheme to replace the decaying Hammersmith flyover with a west London road tunnel.

The Mayor announced Transport for London was now behind plans drawn up by Hammersmith and Fulham council to create a “flyunder”.

Officials at the transport body, which is spending £60 million fixing the 620-metre concrete structure to keep it in use for decades, have previously been sceptical about the proposal.

But Mr Johnson, who previously said the project’s viability depended on the development potential of the land released by the tunnel, said the finances now added up.

He said on his AskBoris radio show on LBC: “We’ve been listening to this for months and months and months and actually it is brilliant. It adds up. It’s the most fantastic scheme.

“We’re going to tunnelise the flyover, the time scale will be three or four years I expect. Even the hardened TfL engineers looked through this, they’ve been pretty sceptical, and they thought it was a great scheme.”

The Mayor claimed there was now “sheer excitement” at TfL about the report commissioned by town hall bosses amid growing concern about the lifespan of the 53-year-old elevated A4 route, which is used by 90,000 vehicles a day.

Three shortlisted options in the study - drawn up with Channel Tunnel engineers Halcrow — include a one mile “cut and cover” version costing £218 million to a two-and-a-half-mile route in a deep tunnel with a £1.7 billion price tag.

The report found that the removal of the flyover and the rest of the A4 through Hammersmith would release prime riverside land that could be worth up to £1 billion.