In September the secretaries of state for the environment and communities and local government granted a development consent order for utility firm Thames Waters’ Thames Tideway Tunnel, which would run from Acton in west London to Abbey Mills in east London.
The project is designed to prevent raw sewage spilling into the Thames following high rainfall.
The scheme is the largest to go through the nationally significant infrastructure projects (NSIPs) process.
But it emerged late last week that the scheme is to face two separate judicial review challenges.
In a statement the Thames Blue-Green Economy said it had lodged a judicial review in the Royal Courts of Justice.
The group said the challenge "concerns the government’s unlawful decision to grant a DCO (Development Consent Order) for the Thames Tideway Tunnel".
It said the decision "is unlawful because the government was in breach of the public participation requirements under the EIA Directive and British Law".
The Thames Blue-Green Economy describes itself as "a broad group of independent experts - including engineers, politicians, lawyers, economists, financiers, journalists, architects, landscape architects, Climate Change scientists, Public Health advisors and environmentalists – whose objective is to halt the Thames Tideway Tunnel project, so that a cheaper, quicker, lower risk, and more sustainable solution to dealing with London’s excess rainwater can be implemented and effective immediately without waiting another 10 years, an option which has been fiercely opposed by government to date".
Thames Tideway Tunnel Ltd said it was aware of the legal challenge by Thames Blue-Green Economy and also of a separate legal challenge by the London Borough of Southwark.
It said: "We are aware that the London Borough of Southwark and ‘Thames Blue-Green Economy’ have filed applications for a judicial review of the government’s decision to grant a Development Consent Order for the Thames Tideway Tunnel. Work continues on the project, business as usual".
The London Borough of Southwark was asked to comment but had yet to do so at time of publication.