The technically demanding contract for the south spur of the £16bn line involves the complex job of refrubishing the Victoran Connaught Tunnel running beneath the Royal Docks.
The move sends out a clear signal that work is proceeding as scheduled on the southern spur of the line.
Recent reports have suggested that spending cuts could see parts of the line or stations scaled back.
Contract 315 is expected to take more than 4 years to complete and involves the technically demanding task of re-excavating and expanding the Victorian tunnel.
The job includes construction of a new tunnel lining through a 105m stretch of the tunnel and surface railway works.
The winning contractor will also need to repair a long stretch of Victorian brick arches running for 500m in the approaches to the tunnel.
Contractors have until 5 July to submit PQQs.
