Bidding has opened for contractors to build the new Lower Thames Crossing tunnel.

The £2 billion package from Highways England is the largest contract ever put out and will see firms vying to become the one to construct Britain's longest road tunnel.

The contract will see companies put forward designs for how they will construct the two massive tunnels carrying traffic underneath the River Thames between Kent and Essex.

The 16-metre wide tunnels, which will go under the river between Gravesend and Medway, will be some of the largest bored in the world and the contract also involves building the portals, approach roads and tunnel systems.

Contracts for the remaining parts of the project – joining the tunnel up with the A2 in Kent and the A13 in Essex – will be put out early next year, highways bosses say.

The overall project, estimated to cost £6bn, will feature 14.3 miles of new road and the two 2.6 mile-long three-lane 70mph tunnels. It is expected works could start as soon as 18 months' time.

Dubbed the country's "most ambitious roads project since the M25 was completed" by the project's executive director Matt Palmer, Highways England expects to whittle submissions down to three contenders and award the contract in 2022.

Matt Palmer, Lower Thames Crossing executive director, said: “At a time of huge uncertainty for the industry, this contract shows our commitment to this project, which will support thousands of jobs during its construction and provide a huge economic boost to the UK economy when it opens for traffic.

"The scheme will relieve congestion at the Dartford Crossing by providing a new free flowing road, almost doubling road capacity across the Thames and supporting sustainable local and regional economic growth."

The Dartford Crossing is way over capacity with 180,000 vehicles regularly using the crossing daily when it was only designed to handle 135,000 per day and take between three and five hours for delays to clear following a closure.

Capacity for the Lower Thames Crossing, which has proved extremely controversial, is estimated to take 74,000 drivers per day in its first year – equal to 27 million in total – which Highways England says will provide "much needed relief at Dartford".

Improvements to the M25 crossing are also planned as part of the Lower Thames Crossing project which saw fresh plans revealed in June.

Keith Bowers, Lower Thames Crossing’s tunnels and systems director, said: “This contract is unparalleled in its ambition, and we need the right partner to match that ambition.

"The Lower Thames Crossing is the UK's most ambitious roads project since the M25 was completed 35 years ago..."

"From our bidders we’re looking for outstanding construction, health, safety and wellbeing performance.

"We have committed to targets that mean by 2040 nobody will be killed or seriously injured on our roads and motorways, and we need our contractors’ design and delivery to meet that target for our road users and workers.

“We are setting priorities in our contracts that will reward excellence during delivery by offering an enhanced share of cost savings for high performance in areas including health and safety, customer focus, delivery, environment, people and communities and economics.”

The project is said to create tens of thousands of jobs during the construction timeframe including labourers, civil engineers, archaeologists, surveyors, caterers, cleaners.

A planning application was submitted on October 23 and officials at the Planning Inspectorate have 28 days to review the application before deciding whether it needs further examination in public.

The transport secretary is due to make a final decision in 2022 with construction planned to begin later that year.

Bidders will be asked to return a selection questionnaire before the end of the year via the Bravo platform.

Designers revealed in the summer less land would be used for the project than initially expected.

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