The cost of the extension has drawn close to £700m
The estimated final bill for the M74 extension through Glasgow and South Lanarkshire has risen by £35m.
According to the financial watchdog, Audit Scotland, the five-mile motorway project will now cost about £692m - more than double the original estimate.
Transport Scotland said 95% of costs were now fixed.
The remaining 5% would depend on final bills for land compensation, utility diversions and supervising the site during the three-year construction.
When the project was commissioned by the Labour/Liberal Democrat Scottish Executive, the cost was originally estimated at £250m.
This was later revised to between £375m and £500m.
In February, the cost was stated as £445m plus an allowance of about £12m to deal with mineworkings.
With the £200m already spent, this took the projected cost to £657m.
But the Audit Scotland report into public construction schemes across the country found that in many cases "early cost and time estimates were too optimistic".
A spokesman for Transport Scotland said the M74 extension would "still deliver a cost-benefit ratio of about five to one for every pound spent".
Work on the five-mile extension officially started in May.
It is expected to open in 2011, connecting the M74 to the M8, west of the Kingston Bridge in Glasgow.