A formal notice to start procurement of a seven-mile "missing link" section of the A8 just east of Glasgow, upgrading it to motorway status, was published by Transport Scotland, along with widening key sections of the M74, M73 and upgrading the Raith interchange at junction 5 on the M74.
But the Government agency admitted it would not have a clear idea of how much the combined scheme would cost the scheme's cost until next spring and construction will only begin in late 2013, with the roads expected to be open to traffic three years later.
The M8 scheme, which will see the dual carriageway section between Baillieston and Newhouse replaced with three lanes in either direction, was due to be completed by 2013 and cost £210million but the costs and timescales have not been formally updated for years.
Transport Minister Keith Brown confirmed that the Scottish Government would seek to attract private finance through its Non Profit Distributing model – a form of PFI in which profits are capped.
He said: "This is a key delivery milestone of this significant project that will complete the last remaining gap in the central Scotland motorway network. On completion, it will further boost the nation's economy by improving connections between the commercial centres of Glasgow and Edinburgh and beyond."
However, Labour expressed disappointment that the project, which is expected to cut journey times on the M8 by 20 minutes at peak periods, will not be completed ahead of the Glasgow 2014 Commonwealth Games in Glasgow.
Michael McMahon, the Labour MSP for Uddingston and Bellshill, said: "This project has already been delayed for three years by the SNP government, which means that before a single inch of tarmac is laid it will already be four years behind schedule."
Glasgow Chamber of Commerce chief executive Stuart Patrick welcomed the announcement but said he was "concerned" that the completion date was 2017.