Four lanes of the motorway in each direction have been narrowed to two over the Woodside viaduct north of the city centre since March – and will now remain coned off for another two years.
The news was described as a “bitter pill for tens of thousands of Scots drivers” by motoring group IAM RoadSmart.
The outer lanes of the 50-year-old viaduct have been shut between junctions 16 and 17 as a safety precaution after it was found to be in a poorer than expected condition.
Amey, which maintains the motorway for Transport Scotland, said the restrictions would be removed once the viaduct had been temporarily propped up to enable its “supporting crossheads” to be repaired.
An industry source told The Scotsman that ensuring no damage was caused to the Glasgow Subway’s tunnels beneath the viaduct was a “particular engineering challenge”.
They also said a “multitude” of underground cables and pipes had been identified, such as gas and water mains and high-voltage electricity, some of which were not shown on plans.
Neil Greig, the Scotland-based policy and research director of IAM RoadSmart, said: “The M8 is worn out and needs this long overdue TLC, but the prospect of another 24 months of lane closures will be a bitter pill for tens of thousands of Scots drivers.
"The impact on the economy and the environment of these measures does not seem to play any part in at least trying to minimise the delays.
"The Scottish Government should be suitably embarrassed that the most abiding memory for many COP26 delegates will be cones and tailbacks.”
An Amey spokesperson said: “Transport Scotland were fully aware from their regular inspection regimes that the viaducts required repairs and undertook testing, inspection and load assessment to allow the full scope of that work to be determined.
"The required repairs were found to be more immediate than originally anticipated and the decision was taken to restrict traffic on the structures to reduce loading until the installation of propping and the repairs could be completed.
"The structures have been inspected annually and to national standards.
"Additional inspections of the half-joints were undertaken on a quarterly basis since 2020 to determine the rate of change.
"As part of this work, poorer than expected condition of the structures became apparent leading to the need for these measures.
“Based on the current programme, the propping works are expected to be complete in autumn 2023, which will allow removal of the current M8 lane restrictions.
"The permanent repairs will be completed in parallel with the propping works, which allow full access for repairs of the supporting crossheads.
"Our key aim in delivering the project is to reduce the overall timescale of the repair and maximise efficiency by ensuring progressive stages of the work overlap.”
Amey said the junction 18 eastbound on-slip, shut with the lane closures, would re-open on Sunday.
Subway operator Strathclyde Partnership for Transport said: “All proposals and assessments to look at the potential effects of the work on Subway infrastructure are being reviewed by our technical support contractors and we are content there should be no impact on the Subway or modernisation programme.”