Design and consultancy work worth up to £110m is up for grabs on the five-year deal.
The contract covers the part of the route being developed by Network Rail.
Transport for London is handling the Central Operating Section between Wimbledon in the south and Tottenham Hale and New Southgate in the north.
Network Rail is in charge of the On Network Sections where the central tunnel comes out onto the existing network in the south west and north east London.
The development phase of the £20bn scheme is expected to start next April.
This contract will cover design and consultancy services for the On Network Sections and is worth an initial £30m-£60m with potential adds-on of £25m-£50m.
Firms must express an interest by September 14 to [email protected]
An online conference call will be held with all bidders on September 23. To register click here