Lanes will provide a range of services including emergency blockage clearance and remedial drainage work across the region.
The Enquirer understands Lanes beat competition from major players including Skanska, Carillion and Morrison Utilities.
Work will start in July (Thames Valley) and August (North London).
Both contracts are for a minimum of three years, with the option to run for eight.
Lanes is currently investing £3m in new plant and equipment to service the contract from a new office in Slough.
Lanes Group director, Andy Brierley, said: “This is our biggest win to date, and comes after a rigorous tender process which took eighteen months of hard work and planning so obviously we are delighted.
“We are now mobilising ready for the start dates, with capital investment of over £3 million in plant and equipment and a new contract head office in Slough.
“Thames Water is the biggest water and sewerage firm in the UK with 14 million customers so it is a huge coup for Lanes to have been awarded these prestigious contracts. The success reaffirms our position as market leader in the UK.”
Thames Water’s Head of Contract Management, Conrad Ashby, said: “The services to be provided in these contracts represent a real step change in our ability to deliver an enhanced experience for all Thames Water customers, to deliver significant operational efficiencies and to significantly improve the knowledge of our asset base.
“This is the start of an exciting journey for Thames Water and Lanes Group plc in delivering a progressive and best-in-class wastewater network service to TW customers.”