The transport secretary has given formal approval to the £9bn Lower Thames Crossing, a road tunnel joining Essex and Kent.

Heidi Alexander granted a development consent order on Tuesday morning, after the decision had been pushed back again last year by the new government.

The controversial scheme has for years been Britain’s biggest single planned road building project, and delays, consultations and redesigns have already cost about £1bn.

The Lower Thames Crossing project will comprise more than 14 miles of roads including the tunnel, which will pass under the River Thames near Thurrock.

The two 2.6-mile tunnels under the Thames will be motorway-sized, with a 70mph speed limit and three lanes in both directions, linking the M25 to Channel port traffic.

The government hopes that private finance will pay for some of the construction, with tolls expected to recoup some of the official £8.95bn “baseline cost” estimate at 2023 prices. Opponents believe the final price tag will be nearer £16bn.

A government source said: “The Lower Thames Crossing will be a key strategic route for drivers, freight, and logistics – improving connectivity between the south and the Midlands, linking up our ports, and unlocking regional economic growth.

“This demonstrates this government’s commitment to delivering the vital infrastructure the country needs to succeed, and to be on the side of the builders, not the blockers.”

The announcement comes on the eve of Rachel Reeves’s spring statement. The chancellor has made clear her quest for growth-boosting infrastructure projects – particularly those that can be funded outside the public purse, such as Heathrow’s third runway.

Proponents say the crossing is vital to take the strain off the congested and unreliable Dartford Crossing, which handles the bulk of traffic east of London including freight from the Channel ports. Dartford’s original design capacity of 135,000 daily vehicle crossings has long been exceeded and traffic is persistently delayed at peak times, even without the frequent incidents or closures.

National Highways, the arm’s length government body that runs England’s motorways and big roads, will be responsible for building the Lower Thames Crossing. It claims that the project will help the UK economy by £200m a year from easing congestion alone.

Matt Palmer, the National Highways executive director for the scheme, said: “It will unlock growth with quicker, safer, and more reliable journeys and redraw the blueprint for building major projects in a net zero future by scaling up the use low-carbon construction, and leaving a legacy of green spaces, green skills.

“Our plans have been shaped by the local community and refined by robust and rigorous examination from independent experts. We are more committed than ever to working with our neighbours to build the crossing in a way that offers them opportunities to work and learn new skills while reducing impacts.”

Work will now continue to set out the preferred funding model, with the Treasury potentially still backing the project in full.

Construction could start as early as 2026, with main works expected to take six to eight years, supporting up to 22,000 jobs, according to the Department for Transport – subject to funding and potential judicial review.

Campaigners who have already successfully challenged road building schemes called the decision “desperate”.

The Transport Action Network director, Chris Todd, said: “This is absolute madness. It’s a desperate decision to distract from the likely bad news in the chancellor’s spring statement tomorrow. When the government can’t afford to properly fund the NHS and our roads are falling apart … It really has got its priorities in a muddle.

“Rather than boosting growth, this will clog up roads in the south-east and slow the economy down even more.”

Proposals for another Thames crossing east of Dartford were first laid out in 2009 by the DfT, and the preferred route of the roads and tunnel was announced by the Conservative government in 2017.