The transport secretary has given formal approval to the controversial and long-delayed £9bn Lower Thames Crossing, Britain’s biggest road-building project.

Heidi Alexander granted a development consent order on Tuesday morning to the new road tunnel under the Thames joining Essex and Kent, after the decision had been pushed back again last year by the new government.

The scheme has been officially backed but shrouded in uncertainty for more than a decade, and consultations and redesigns have already cost almost £1bn.

Business and logistic groups welcomed the decision but environmental campaigners said it would worsen pollution and congestion.

The project will comprise more than 14 miles of roads including the 2.6-mile crossing under the Thames near Thurrock, Essex. It is made up of two tunnels each carrying three lanes of traffic in one direction with a 70mph speed limit, linking the M25 to Channel port traffic.

The government hopes 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 first opened in 1963 and 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 of disruption or closures.

National Highways, the arm’s length government body that runs England’s motorways and main roads, will be responsible for building the Lower Thames Crossing. It claims 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 of low-carbon construction and leaving a legacy of green spaces, green skills.”

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 (DfT) – subject to funding and any potential judicial review.

John Armitt, the chair of the National Infrastructure Commission, said the project’s approval was “excellent news and will be welcomed not only by local drivers and communities but also by businesses across the country which depend upon reliable access to Europe, but are often frustrated by delays at the Dartford tunnel”.

The Logistics UK chief executive, David Wells, said: “Industry is united in its backing for this vital trade route [which] when completed will unlock UK logistics, drive growth and help keep supply chains moving across the whole country.”

But 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.

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

Ben Plowden, the chief executive of Campaign for Better Transport, expressed disappointment, adding: “We know building new roads increases carbon emissions and fails to relieve congestion, and the Lower Thames Crossing will be no exception. We fully accept the need to meet the transport challenges of south-east England, particularly cross-river connectivity, but an expensive road tunnel is not the way to do it.”

The Green party MP Siân Berry described it as “a gargantuan waste of money”, adding: “How many years before a multibillion-pound relief road is suggested to cut congestion on the Lower Thames Crossing by those who want this scheme?”

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.