An east London road tunnel will cost £200 million more than the £1 billion previously quoted, documents have revealed.

Silvertown tunnel will cross the Thames from the Greenwich peninsula – due to be built by private contractor Riverlinx, the new road is funded by Transport for London (TfL).

The transport network and the Mayor of London have said the scheme will cost £1 billion to build.

But the contract award notice, published on Monday, puts the total cost at £1.2 billion – revealing extra spending outside the construction costs that add 20 per cent to the project’s price tag.

TfL says the build cost has not increased, and the extra £200 million covers the cost of planning applications and interest Riverlinx will have to pay on money it has borrowed for the scheme.

A spokesperson for the transport authority said: “There have been no increases in the overall price of the project, and the basic construction value is around £1 billion as we have always said.

“The figure referenced in the contract includes expenditure already outlaid by TfL, plus the cost of debt up to the forecast opening date in 2025.”

Backed by the Mayor, the multi-lane road aims to reduce congestion on both sides of the river.

But residents fear the road will bring more traffic and increase pollution – and most borough councils in the area

TfL signed the contract for the tunnel in November, and the Mayor has been criticised for green-lighting the controversial project ahead of the Mayoral election in May.

A new Mayor opposed to the tunnel could face high costs to cancel it.

But a TfL spokesperson said the price of the scheme was now “locked in” and Riverlinx would “bear the brunt” of any additional spending.

The contractor will shoulder the cost of the tunnel until it is open, and TfL will then pay Riverlinx back over a 25 year period, with payments contingent on the tunnel being open and in use.

The spokesperson said £1.2 billion was the maximum amount the transport network would have to repay, unless there were changes to the contract – which he claimed “won’t happen”.

But Liberal Democrat assembly member Caroline Pidgeon, a long-standing opponent of the tunnel, said she expected costs to “escalate even further”.

She said: “Of course the project’s real cost will be even greater because of its impact on the environment.

“It is a scheme that will generate more road traffic and inevitably more pollution.

“There are real alternatives to building an expensive motorway under the Thames.”

Green Party co-leader and London Assembly member Sian Berry said she had demanded clarity on the cost of the tunnel from the Mayor as recently as last week.

She said: “Now we learn that even his plans cost £200 million more than the most recent estimate, bringing the total that would be wasted on this scheme to an eye-watering £1.2 billion.

“The Silvertown Road Tunnel is a dinosaur project that isn’t embracing the future London needs – one of low-carbon public transport, walking and cycling that helps people get about without clogging the roads for miles around and choking the air with vehicle fumes.”

Victoria Rance, spokesperson for local protest group Stop the Silvertown Tunnel Coalition said she was “furious” that money would be “sucked out of the local economy” to fund the project.

She said: “This massive new piece of fossil-fuelled infrastructure that will increase greenhouse gas emissions, and worsen already dangerously polluted air for some of London’s poorest communities.”

She added: “This scheme is a kick in the teeth for all the local boroughs who have pleaded with the Mayor not to go ahead.

“They need evidence-based and effective action on carbon and pollution reduction right now, not a dirty, expensive and ineffective scheme that will take five years to build.”