Campaigners against £1billion plans to build the Silvertown Tunnel have threatened to take the Mayor of London to the high court.
Members and supporters of the Stop the Silvertown Tunnel Coalition (SSTC) warned Sadiq Khan that if he allows TfL to sign the contract they will ask a judge to review the decision.
Victoria Rance, from the SSTC, said: "If Mayor Khan won't think again, the Silvertown Tunnel must have a judicial review. Assessments show pollution will worsen for thousands of our school children and residents.
"And it completely fails on global warming. We need new public transport, cycling and walking investment. Road building and business as usual will kill us. The rich can escape flooding, congestion and pollution. The poorest can't."
She claimed the mayor is aware TfL made errors in an initial analysis of the venture with climate concerns "scoped out" of studies altogether.
A spokesman for the mayor said: "It is essential we continue with plans for more river crossings in east London. The construction of a new road tunnel at Silvertown - combined with the introduction of tolls on the Blackwall Tunnel and at Silvertown - will play a vital role in tackling congestion, improving air quality and provide additional bus services across the river.
"Extensive modelling on the impacts on traffic, air quality and carbon emissions has been undertaken and has been subject to rigorous peer review and thorough examination in public.
"TfL is in the process of concluding negotiations with Riverlinx and contracts are due to be signed shortly."
The letter states campaigners will seek a judicial review on the basis the decision allowing the contract to be signed is based on information the mayor and TfL "know is insufficient to make a well-informed decision".
The signatories question whether a thorough assessment of all options at the Blackwall Tunnel was made before the scheme was given the green light.
A TfL spokesman said: "The Silvertown Tunnel scheme was subject to significant scrutiny as part of the development process, including a six-month public enquiry."
Riverlinx's bid to carry out the work joining the Greenwich Peninsula to Silvertown had stalled after rival bidder Silver Thames Connect challenged the procurement process.
But the automatic legal halt was lifted in October, meaning the scheme could proceed.
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