Costs of the new Lower Thames Crossing have increased by more than half before plans have been submitted.

The dual-tunnel 14-mile route under the Thames near Gravesend was initially estimated to cost £5.3 billion but latest projections now put the cost at up to £8.2bn.

This is 54% more than first expected and more expensive per mile than the controversial HS2 rail line.

Estimations put the cost of the LTC at £573.5m per mile compared to £307m per mile of track for the high speed connection between London, Birmingham, Manchester and Leeds – which is expected to cost around £100bn in total when complete.

Highways England (HE) says the two projects are not comparable on a mile-by-mile basis as a "significant proportion" of the LTC cost is "building one of the largest diameter tunnels in the world".

Figures issued in the government's Road Investment Strategy 2 document – published in March – detailing the increased cost of between £6.4bn and £8.2bn were not publicised by HE.

Opponents from the Thames Action Crossing Group (TCAG) say it was "strange" there had not been an official announcement relating to the cost despite numerous others during the year.

TCAG chairman Laura Blake said: "It would be £8.2bn of taxpayers' money that would be spent on LTC, and it is public/taxpayers' perception that HS2 has been highly controversial due to the cost and environmental impacts.

"To know that LTC would cost more per mile is a serious concern to us and everyone we have spoken to. Nearly £573.5m per mile for LTC is an extremely large amount of taxpayers' money, especially considering that it won't solve the problems we all suffer with due to the Dartford Crossing.

"On the Kent side, when there is an incident at the Dartford Tunnels and traffic needs to migrate to the LTC, there is only one single lane from the A2 coast bound onto the LTC. Imagine the chaos, congestion and pollution that would create.

"There are other examples of similar issues along the route both sides of the river."

The government has reiterated its commitment to LTC with Chancellor Rishi Sunak including the tunnel in his Spending Review announced last week.

It comes as a parliamentary inquiry looking into the future transport needs of the country as a result of the Covid-19 pandemic will be assessed.

Chairman of the all-party parliamentary group on transport Huw Merriman (Con) told The Observer on Sunday that transport funding is at "a pivotal moment".

He said: "With so much uncertainty, pressing the accelerator too early could lead to vast transport infrastructure projects which are either not needed or are sited in the wrong parts of the country to level up."

One of the areas expected to be investigated is the £27bn roads building strategy – of which almost a third is made up of the LTC budget.

The inquiry will start hearing evidence from Wednesday.

TCAG says the cost of the new tunnels is not justifiable and claimed information released by HE has been "misleading" around the traffic capacity of the crossing.

Ms Blake says the group holds reservations about the true economic benefits claimed by HE, inadequate consultations, and the lack of information leading to the Planning Inspectorate indicating it would be refusing to accept the submission of the planning application.

Spokesman for Highways England Harry Bellew said: "We welcome the news that the project remains a priority for government.

"The Lower Thames Crossing is the biggest and most complex road scheme since the M25 opened almost 35 years ago.

"It is normal on projects of this scale to update cost estimates as we continue to refine and improve the scheme to deliver the right solution for the local communities, the environment and the taxpayer.

"As the design develops, we will have more certainty of the costs.

"The Lower Thames Crossing will improve the journeys for millions of road users, support tens of thousands of jobs, and create a new connection that will bring billions of pounds of economic benefit to build back the post-Brexit and post-Covid economy."

Days before HE withdrew its plans, managers put a £2bn contract out to tender for firms to design and construct the massive new tunnel.

Despite the planning documents being pulled at the eleventh hour, HE says the tender – the largest the company has ever published – is still active. The remaining roads contracts for the Essex side and the A2/M2 sections are due to be announced next year.

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