The four consortiums have been set a deadline of 27 February for receipt of their final offers. The SRO expects to select a preferred bidder by early May.

A decision had previously been anticipated during the first quarter of 2008, but the best and final offer process was added late in 2007 following discussions with the bidders over the legal agreements within the final contract.

Some bidders have expressed concern over the length of the 30-year concession and the difficulties of running such a large rail project profitably on a build-operate-transfer basis.

Advisers to the SRO say these concerns have since been resolved.