This was despite an unresolved complaint by Green Party members to the European Commission that the award was anti-competitive and illegal under European Union procurement legislation.
Interlink M74, the sole bidder, was awarded the job on a fixed price of £445M plus an allowance of £12M should treatment works to mine workings along the corridor be required.
However, a formal complaint by Scotland's Green Party is currently lodged with the European Commission over the Scottish government's decision to ditch the original tender for two contracts in 2005 and retender it in 2006 as one scheme.
In the original tender, Sir Robert McAlpine, Vinci and Morgan Est had bid against Balfour Beatty and Morrison for both phases, while Laing O'Rourke bid against both teams on just one phase of the scheme.
The complaint alleges that since it was unlikely Laing O'Rourke would bid for the re-let combined contract, Interlink M74 members had "a good deal of confidence that they would be unlikely to face further competition in the event of them recombining as a single
Following this complaint, the Scottish Government carried out a review of the procurement and concluded it was entirely acceptable under European law.
Green MSP Patrick Harvie described both the procurement and its review as a sham.
"A sham tender process has been concluded with a sham review and an entirely irresponsible decision given that the
European Commission may pull the plug on the process altogether," said Harvie.
Transport Scotland will manage the project which is currently thought likely to begin in May 2008. Completion is due at the end of 2011.
A Transport Scotland spokesman told NCE: " As with all major infrastructure projects legal advice was sought at every step in the process. However, we are happy to co-operate fully in order to clear any doubts created by the recent complaint to the Commission."
With over £180M already spent on the scheme, mostly in compensation payments to businesses along the route, the total cost is likely to be £650M.