Ministers have just over a month to finalise a deal to build the half-billion-pound "missing link" in Scotland's motorway network, it emerged last night - as doubts persist over the government's commitment to the project.

Senior officials are in sensitive talks with the sole consortium to bid for the right to construct the M74 extension, effectively completing a high-speed ring road around central Glasgow first planned in the closing months of the Second World War.

The Scottish Government and the consortium, Interlink M74, have just 90 days to thrash out agreement on a bid, which, The Herald understands, was formally lodged in early November.

Last night supporters of the development reiterated the case for the road being built.

George Redmond, a councillor in the east end of Glasgow and a lead figure in the Clyde Gateway project, a £1.4bn regeneration scheme, said: "Why would you build a 400-mile motorway and not finish the last five miles? This extension is critical to the economy, giving businesses easy access to the rest of the UK. It will also drive down congestion on other roads and replicate the M77 extension in cutting accidents."

The Herald today sets out the overriding economic case for the motorway, which is expected to bring an extra 20,000 jobs to Scotland's poorest communities in two decades.

Insiders said government officials had already secured concessions from the builders but it is not known how high a price Interlink M74 was able to name.

The M74 Completion - or M74C for short - has been officially costed at between £375m and £500m, including a robustly forecast £180m in compensation for businesses along its route.

Bids were sought for the construction work to be done for around £275m. Inflation in the building sector, however, is high, partly because so much of the UK construction industry's capacity is caught up in projects related to the London Olympics.

Last month there was speculation that the new SNP administration may delay the M74C. Sources, however, last night said SNP ministers were every bit as convinced in the need for the M74C as their predecessors.

A spokeswoman for Transport Scotland said last night: "The final bid is in and we are carefully considering this before presenting our findings to ministers. We expect the project will be completed by 2011."