A controversial four-lane road tunnel under the River Thames will cost nearly £2bn over the next three decades if it goes ahead, according to accounts published by Transport for London.

The figure – which includes the construction, maintenance and operation of the tunnel as well as interest payments on the debt – is more than twice the original estimate for building the tunnel.

Critics say the new figure, revealed in “a small, discreet note” in TfL’s published accounts, proves the cost of the project was spiralling out of control.

Caroline Russell, Green party assembly member, said: “When my predecessor on the assembly first started opposing this scheme Transport for London described the cost as being about £600m. We are stratospheres above that now and TfL will be saddled paying for this, even in our uncertain future.”

Russell also accused the London mayor, Sadiq Khan, of trying to hide the full cost of the scheme.

“Last year he told us the Silvertown road tunnel would cost £1bn, but now we finally see what TfL will actually pay and it’s heading for £2bn. This is outrageous.”

Transport for London said the existing cost of building the tunnel – around £1bn – had not increased. It said the additional costs covered in the £2bn figure included “ongoing operational costs … for 25 years as well as any maintenance and asset renewal required”.

There has been growing opposition to the project from local residents and green campaigners who say it would be environmentally destructive and undermine efforts to clean up the capital’s polluted air. They also argue it “fatally undermines” efforts to build a green recovery from the coronavirus – with dire consequences for some of London’s most vulnerable citizens.

Last week the shadow minister for climate change and local MP, Matthew Pennycook, added his voice, calling on Khan to abandon the scheme, saying “green transport must be the future of our city”.

If it goes ahead, the tunnel would be funded with a loan to be repaid by introducing tolls on the Blackwall and Silvertown tunnels.

The TfL accounts state the authority would repay £65m a year – index linked – between 2025 and 2050. At 1.6% [the current retail price index], that would lead to a total payment of about £2bn by 2050.

A Transport for London (TfL) spokesperson did not dispute the overall figure of £2bn but said the cost of building the Silvertown Tunnel – around £1bn – had not increased.

In a statement they added: “All payments towards the Silvertown tunnel will be funded from a road user charge, which will cover both the Blackwall and Silvertown Tunnel and will only begin once it is available for use, with payments linked to the performance of the contractor on key criteria such as availability.

“These charges will not only repay the construction costs, but will also cover the ongoing operational costs for the tunnel for 25 years as well as any maintenance and asset renewal required.”

A spokesperson for the mayor said Khan “ has been clear that he doesn’t want to replace one health crisis with another, and he is determined that our city’s recovery from coronavirus will be clean, green and sustainable”.

The spokesperson said a new tunnel at Silvertown was crucial because the existing infrastructure was “antiquated and worn out”.

“The tunnel will provide a public transport-focused river crossing with improved bus links across the Thames. The combination of introducing tolls on both the Blackwall tunnel and at Silvertown – and the extension of the ultra low emission zone from 2021 so that Silvertown will be within the area covered – will play a crucial role in tackling congestion and improving air quality.”

Steve Chambers, an independent transport consultant and academic at London South Bank University, said the £2bn price tag meant the scheme was an “expensive debt” for TfL to service.

“The financial risks of the scheme are becoming clearer and there is no guarantee that the debt can be serviced from tolls alone. Future mayors and Londoners could be left to pick up the tab with other more sustainable transport plans delayed or abandoned in order to keep up with repayments.”