Transport Scotland has now been forced to delay signing maintenance contracts worth £230m.
An Amey bid was ruled out earlier this year because it was regarded as ‘abnormally low’. Amey is a subsidiary of Spain's Ferrovial.
A spokeswoman for Transport Scotland said: "We can confirm a bidder put in a price that was relatively low. The bid was thoroughly investigated and found to be abnormally low, carrying unacceptable risk. Therefore a decision was made to reject this bid.
"We will robustly defend any legal challenge that is brought forward but are unable to comment further at this time."
Amey, which currently maintains more than 430 miles of trunk roads and motorways in the southwest of Scotland, lost out to Scotland Transerv, which takes over responsibility for the routes in March 2013.