The latest section of the Heads of the Valleys road will not be completed until April 2021, the Welsh Government has announced.
Eight kilometres of road is due to be turned into a dual carriageway through the Clydach Gorge in Gwent.
The work will have taken more than six years to complete, when it was due to last less than four.
The Economy Minister Ken Skates said he is "obviously very disappointed by the further delay".
I have asked the project team to continue to explore what can be done to bring forward these programmed dates. However, the scheme budget [has seen] no further increase despite this latest delay.
The cost originally estimated for the Heads of the Valleys road
Construction started at the end of 2014, when the road was due to be completed by autumn 2018.
It is now expected to overrun by two and a half years, to spring 2021.
The cost was originally estimated at £223 million, but by last November, the estimate had risen to £321 million.
But the final cost to the taxpayer is still unknown, as the Welsh Government and the contractors, Costain, have taken a financial dispute to arbitration.
One argument, over the project’s design, was settled last year when they were told to split a bill of approximately £40 million. Costain’s annual profits were halved as a result.
Constructing a dual carriageway between Brynmawr and Gilwern, near Abergavenny, was always going to be an engineering challenge, with a series of difficulties to overcome including:
The route is through the Brecon Beacons National Park and home to protected species like bats and rare varieties of trees.
The geology is unstable, with three different types of rock and extensive unmapped mine-workings, dating back to the eighteenth and nineteenth centuries.
There is also a major cave system, for the most part equally uncharted.
The narrow gorge drops by 270 metres and contains a main sewer, which had to be diverted, as well as a high-pressure gas main.
There isn't enough room for a completely new route, so the existing single carriageway runs through the middle of the roadworks.
21,000 vehicles a day pass through the construction site, with traffic lights not allowed at peak times and road closures only permitted at weekends.
These problems were known to the Welsh Government when it planned the improvement to the A465 Heads of the Valleys road and the construction firm Costain similarly signed the contract.
A special ‘maternity roosting building’ for lesser horseshoe bats was built at the start of the project and a rare Welsh Whitebeam tree was carefully replanted away from the route.
But there were still some surprises, ranging from the discovery of asbestos on a former industrial site at the Brynmawr end of the route to finding 67 graves at the former Primitive Methodist chapel in Gilwern.
Particularly expensive was that part of the embankment carrying the existing road was slipping and required extensive pilling and stabilising work.
The original Heads of the Valleys road was built in 1960s and known at the time as “Marples’ motorway on the cheap”, after the Minister of Transport who decided that a single carriageway was enough. The Auditor General for Wales says lessons must be learnt from today's far more expensive project.
This is not the first time that the Welsh Government has faced difficulties with significant cost increases and delays on road projects and it is vital that lessons are learnt for future infrastructure schemes. Despite some wider benefits being delivered and expectations about the eventual impact of the road improvement, those living and working locally are paying a higher than expected price for the ongoing delays and disruption during construction.
A Welsh Government spokesperson responded by saying that the project is widely recognised as one of the most complex road engineering projects in the UK at the moment, presenting significant construction challenges. The spokesperson added that the Welsh Government recognised the frustration caused by the issues highlighted by the Auditor General.
Lessons from the project so far have already been incorporated into subsequent projects. We remain confident that this ambitious and complex scheme will be completed as intended and will bring significant benefits to the region.