Crossrail is in the process of negotiating Mitchell’s leaving date.

Mitchell will report to the Thames Tideway Tunnel’s recently appointed chairman, Sir Neville Simms, and lead the development of a new company that would be responsible for the financing and delivery of the landmark project. The process for the procurement of this new company is already in motion, following the issue of a preliminary notice to the market on 6 January 2014.

Simms said: "The Thames Tideway Tunnel must continue to showcase all that is best about Britain’s capability to deliver complex civil engineering projects, on time and to budget. In Andy, we are privileged to have the services of a highly-skilled individual, who has been instrumental in doing this for Crossrail. His skills and experience are a natural fit for our project. I am personally thrilled to have him on the team."

Mitchell added: "As an engineer, I am passionate about the critical place well-functioning infrastructure has in driving prosperity and growth. Just like Crossrail, the Thames Tideway Tunnel is essential to the long-term social and economic well-being of the capital and the country. I am honoured and excited to have this opportunity to be in the vanguard of making the project a reality at last.

"Having been closely involved in its establishment at Crossrail, I am particularly looking forward to developing the Tunnelling and Underground Construction Academy at the Thames Tideway Tunnel, ensuring the country has a ready and available pool of engineers capable of meeting the infrastructure challenges the country faces over the next few decades."

One of the UK Government’s top 40 national infrastructure projects, the Thames Tideway Tunnel, is required to help tackle the discharges to the River Thames of untreated sewage from London’s overstretched, Victorian sewerage network. Currently, as little as 2mm of rainfall can trigger a discharge to the river’s tidal stretches through central London.