The central package of work sent out to tender is worth between £600 million and £950 million and the successful bidders are expected to be announced in May 2015.
The project has been split geographically into three main construction works contracts, with the invitation to tender for West sent out in December 2013 and East in January 2014.
Overall the packages are worth up to a total of £2.3 billion.
Thames says the tunnel will help tackle the problem of overflows from the capital’s Victorian sewers and will protect the tidal River Thames from increasing pollution “for at least the next 100 years”.
The Development Consent Application is currently being considered by the Planning Inspectorate and, subject to consent being granted, construction on the project is due to start in 2016 and will involve 24 construction sites across London.
The project will be financed and delivered by an independent Infrastructure Provider (IP), with its own licence from regulator Ofwat.
Mike Gerrard, Thames Tideway Tunnel managing director, said: “We are making excellent progress with the tender process and we are confident that we will receive competitive bids that will result in the best value for our customers.”
Last week Thames Water had its outlook downgraded to negative by ratings agency Moody’s due to concerns surrounding the project.
Moody’s said the company might not be able to absorb the financial risk posed by its involvement in the tunnel. If senior creditors approve Thames’ amendments, the company would be exposed to “reputational and financial challenges that other companies in the sector do not face”.