National Highways is in the final stages of improving the junction where the A454 Black Country Route/Wolverhampton Road meets the M6 at Junction 10.
Bosses of the £78 million project - which has involved replacing the bridges on the roundabout over the motorway with structures that can carry four lanes of traffic instead of two - were hoping it would be completed this autumn.
However, the agency has said that workers have "encountered some new and unexpected challenges" that have "significantly impacted" the schedule.
National Highways Programme Leader, Ashfaq Hussain, said: “We expect the majority of the works to be completed in the new year with the upgraded layouts for the Junction 10 roundabout, Black Country Route and Bloxwich Lane junction set to open in January bringing significant benefits for road users.
“Work on the upgrade of the junction has been impacted in recent weeks by a number of unexpected, external factors. These have included national supply issues with some of the signage technology and a delay regarding installation of telecommunications cables. However, despite the challenges, we have finished the majority of the construction work.
“We understand roadworks can be disruptive and appreciate people’s patience while we complete these much-needed improvements which will tackle congestion and bring safer, more reliable journeys.”
The route was described as being a notorious bottleneck with congestion and large queues being a familiar site around the junction which prompted the works. But it hasn’t all been plain-sailing for the scheme, which has taken over two-and-a-half years – and was originally due to be completed in summer this year.
National Highways chiefs say the delay is down to “factors out of our control” citing Covid-19, staff and material shortages and a sub-contractor going bust.
Over the autumn the new roundabout has been resurfaced, new permanent traffic lights have been installed and an environmental barrier on the A454 Black Country Route has been erected. On top of that, overhead gantries on the roundabout and the westbound A454 Wolverhampton Road have also been erected while work on installing permanent vehicle restraint system barriers along the roundabout and slip roads, and pedestrian guardrail and tactile paving at crossings has begun.
Although cones are due to be cleared away on the roundabout and Black Country Route/Wolverhampton Road in January, cones and the temporary 60mph speed limit will remain on the M6 beyond that date while the agency moves to the "technology commissioning phase" of the project.
A National Highways spokeswoman said there was no confirmed date for when the commissioning will be completed, saying it "depends no how the testing goes".
She added: "But all of the work including the commissioning of the technology, landscaping and all final touches is due to be completed by early spring."
Overnight closures will take place until Sunday. On Thursday and Friday nights there will be lane closures at the junction of Wolverhampton Road, Bloxwich Lane and Tempus Drive between 10pn and 5am. There will also be a full closure of the A454 Wolverhampton Road westbound from Pleck Road to the M6 Junction 10 roundabout during the same hours on Saturday and Sunday nights. Traffic will be diverted along Pleck Road to the motorway and A454 Black Country Route.