Sadiq Khan’s “green” credentials were under threat today after plans for a walking and cycling bridge across the Thames were shelved a day after a £1bn road tunnel received his personal endorsement.

City Hall announced the proposed 180m-wide bridge linking Rotherhithe and Canary Wharf, backed by 93 per cent of Londoners in a Transport for London consultation, was being “paused” after estimated costs rose to £463m.

Only £350m had been set aside in TfL’s business plan. There were fears the bridge, which would ease dire overcrowding on the Jubilee line and meet his “healthy streets” policy of encouraging walking and cycling, could exceed £600m.

It came a day after the Mayor revealed his determination to build the Silvertown tunnel, which critics have dubbed a four-lane motorway and environmental catastrophe.

Heidi Alexander, the deputy mayor for transport, said the bridge was “unaffordable in the short to medium term”. TfL is now considering introducing a high-speed ferry service instead.

Ms Alexander said carrying out preparatory work on the bridge – costing about £800,000 a month – would have been “irresponsible” when there was a “low likelihood” of the project advancing in the near term.

In a letter to Florence Eshalomi, Labour chair of the London Assembly transport committee, Ms Alexander wrote:

I am writing to inform you that today the Programmes and Investment Committee of the Transport for London (TfL) Board has agreed that TfL should pause development work on proposals for a walking and cycling bridge between Canary Wharf and Rotherhithe. The committee has concluded that the project should revert to the feasibility stage of development where strategic alternatives, such as a ferry service, can be reassessed. Despite considerable effort to minimise the costs of a bridge at this location, the sheer scale and complexity of the engineering solution that would be required means it is currently unaffordable. The current midpoint cost estimate for the scheme is £463m, within a range that means final costs could be over £600m. This compares to a £350m allocation in the current TfL Business Plan. The bridge is therefore unaffordable in the short to medium term, particularly in the context of TfL’s wider financial challenges. Whilst various financial pressures will need to be managed as part of TfL’s business planning process this autumn, I believe it is right to make this decision now given the significant costs of keeping the project going. To continue development of the scheme through the next phase of surveys, ground investigations and engineering design would have cost around £800,000 a month. It would have been irresponsible to incur these costs given the low likelihood that it will be possible to take the scheme forward in the near term. By making the decision at this point, it has been possible to avoid commencing a number of expensive tasks that were due to start imminently. TfL has done everything possible to explore options for making this project viable. Considerable effort has been put into exploring options for lowering costs through different design options and value engineering. This has included looking at different bridge options, including the potential for a lifting, swing or bascule type bridge, the latter two of which would have been unprecedented in scale globally given the opening requirements for shipping on that part of the river. TfL has also assessed possibilities for additional alternative funding sources, which unfortunately are not adequate to address the funding gap. As you will appreciate, stubbornly high costs also have an impact on the quantified business case for the scheme. Looking forward, it is my hope that we are able to develop a ferry option that is more affordable as a short to medium-term way of providing the walking and cycling connectivity that is needed at this location. TfL will be assessing all options for a ferry service, including a roll-on/roll-off style service using electric or hybrid vessels. This would be considerably cheaper than building a lifting bridge, and the service could be up and running more quickly. The valuable work that has been done to date to demonstrate the technical and operational feasibility of the bridge will be taken to a logical end point to ensure it can be used in the future should circumstances allow. I understand that this is a significant decision and that it will be disappointing for those people – like me – who were excited about what a bridge could do to enhance opportunities for walking and cycling in a part of London which has been historically poorly served in this regard. This is still an important objective and I look forward to continuing to work with the local boroughs, not only in providing a new ferry service as soon as we can but also in constructing Cycleway 4, for which we confirmed a construction start date this week, progressing new routes between Rotherhithe and Peckham and Hackney and the Isle of Dogs and supporting the expansion of Santander Cycles. I would be happy to meet members of your committee or come to a committee session to discuss this matter in more detail and would suggest that David Rowe from TfL accompany me. Given likely media and community interest in this matter, I am making a copy of this letter publicly available.

Matt Winfield, London director at Sustrans, the walking and cycling charity that first proposed the bridge, said: “We are hugely disappointed that the Rotherhithe to Canary Wharf bridge has been cancelled. With so much development planned for east London, it is essential that new walking and cycle crossings over the Thames are built so that it is easier for people to reach jobs and services using sustainable, clean modes of transportation.

“Cycling and walking are a vital part of the transport mix but not if we force people to share tunnels with motor traffic. London needs significant change to address the problems of air quality, fairness, congestion and health – the cancellation of this bridge takes exactly the opposite approach.”

Today it was revealed by Tower Hamlets councillor Andrew Wood that a roll-on, roll-off ferry service could be launched in a few years. He tweeted a picture:

Nicola Brittain, an Extinction Rebellion campaigner, said: “Financial costs aren’t the only costs the Mayor should be considering when deciding to shelve or go ahead with projects. During a climate emergency where we face certain catastrophe if we don’t make systemic changes, environmental degradation as well as the health and wellbeing of Londoners should be central to any analysis. They don’t seem to be, and this is very worrying.”

Keith Prince, GLA Conservative transport spokesman, said: “Since the very start the Rotherhithe bridge has been an expensive and ill-considered pipe dream. Sadly, Sadiq Khan has spent more than £12 million of taxpayers’ money to find that out.

“It’s notable that Peter Hendy, the former Commissioner of TfL, said in January 2017 that ‘the business case for the pedestrian bridge in Canary Wharf to Rotherhithe isn’t much cop.’

“It’s important that there is a crossing at Rotherhithe and I am pleased the Mayor is finally taking the much better option of a ferry service seriously. A roll-on roll-off ferry is a cost-effective, simple and quick plan to create a much needed crossing at an underserved point of the Thames.”

Caroline Russell, Green Party member of the London Assembly, said: “This shocking bridge cancellation may yet have a silver lining for people walking and cycling if the Mayor and TfL move fast on alternative schemes.

“A fraction of the £350 million set aside for the Rotherhithe bridge could be used for a much cheaper ferry shuttle service. It’s my understanding we could be boarding these ferries on foot or by bike in four years if the Mayor and TfL work fast to build new ferry piers with relevant partners – like the Hilton and the Canary Wharf Group.

“The pot could also be used to fill the £14 million funding gap for the Diamond Jubilee bridge – a shovel ready scheme that has been crying out for more money from the Mayor. If he helps with this, a much needed walking and cycling bridge in west London could be open as early as 2021.

“I will be pressing the Mayor and TfL to get going on these good, people-friendly river crossings, and to make sure the ferry shuttle service is free, Londoners wouldn’t expect to pay to walk across a bridge and this new ferry should honour that.”

The bridge would have been the world’s tallest vertical lift bridge at 90m to allow ships to pass underneath.

The Mayor’s spokesman said: “TfL have used all of their expertise to try and lower the costs of a viable new bridge at this site, but it would now cost substantially more than the money allocated in the business plan. Pausing work is now the sensible and responsible thing to do to protect the London taxpayer.”

Mr Khan said the twin-bore Silvertown tunnel was needed to ease congestion at Blackwall tunnel. Contracts are due to be signed this summer, with work completed by 2025.

TfL wants to fund it via a PFI scheme, with drivers paying a toll (amount to be decided) to use both the Silvertown and Blackwall tunnels.

Mr Khan told the London Assembly yesterday: “The current situation with the Blackwall tunnel cannot be allowed to continue.

“It was closed at least 700 times alone in 2017/18, making it the least reliable river crossing in London. The Silvertown tunnel is the best way to relieve this problem.”

Green assembly member Caroline Russell called the Silvertown tunnel a “dinosaur project” and a “climate failure”.

Lib-Dem Caroline Pidgeon accused the Mayor of hypocrisy for being prepared to sign the Silvertown contract only five months after declaring air pollution in London to be a “public health emergency”.

She said afterwards: “His rhetoric on air pollution and climate change is not matched by his decision to build a new road tunnel in London.”

The bridge was included in the five-year TfL business plan. This is how the TfL website outlines the benefits of the bridge:

We want to build a new river crossing for pedestrians and cyclists between Rotherhithe and Canary Wharf. This will create a much needed new route across the Thames in east London and benefit communities on both sides of the river. It will link into nearby cycle routes and help encourage more people to walk and cycle in line with the Mayor’s Healthy Streets approach for London. The proposal forms part of the Mayor’s wider package of river crossings and new walking and cycling infrastructure in east London as set out in his Manifesto and the Mayor’s Transport Strategy. A new river crossing would give thousands of people a direct, above ground link between Canada Water and Canary Wharf, and support jobs and new homes in the area.

An edited version of this story appears in tonight’s Evening Standard.