The roadworks on the A11 have been hit by months of delays, with officials blaming the weather for the hold-up.
The project was due to finish last month, but National Highways has now revealed it will not be completed until at least September.
The organisation has said heavy rainfall over the winter, as well as last July's heatwave, are to blame for the delay.
It means that traffic congestion through the roadworks will last throughout Norfolk's summer tourism season.
The news has caused anger for motorists who rely on the busy route, as well as those living in nearby communities which have seen rat-running from drivers looking to avoid the road.
Suzanne Nuri-Nixon, mayor of Wymondham, said: "Personally, I travel down to London a lot to see my family and it has become a bit of a lottery as to which exit I need to take.
"I know there has been extra traffic down Harts Farm Road and in some of the estates as well, as drivers look for ways to get around the roadworks, while Hethersett has suffered as well."
John Fuller, leader of South Norfolk Council, described the delay as "disappointing".
The £65m project, which started in May 2022, involves the resurfacing of the road between Spooner Row and the Tuttles Interchange at Wymondham in both directions.
During the work, carriageways in both directions have been reduced to single-lane contraflows, with 40mph speed limit restrictions.
While the A11 has remained open for the most part, there have been occasions, including on some nights, when it has been closed and drivers have had to take lengthy diversions.
Slip roads, including those serving Wymondham and Spooner Row, have been closed off at various stages of the work.
A spokesman for National Highways, the government company which is responsible for A roads, said: "The key reason for the small delay in the completion of the scheme is weather.
"We originally aimed for a mid-summer completion date, but we set that target in January 2022.
"Since then we have had an extremely hot summer and a wet winter which has contributed to slower-than-expected progress.
"However, we are on track for a mid-September completion."
READ MORE: Why are there roadworks on the A11?
Last summer was extremely hot, with record temperatures in parts of the UK. But East Anglia remained in official drought status throughout the winter just gone.
The work National Highways has done has seen the concrete road surface, along with some of the foundations, completely removed.
The road was then rebuilt and it was resurfaced with asphalt.
National Highways says the scheme - which remains on budget - will improve safety, create a smoother surface for vehicles to travel on and will reduce noise for the 45,000 drivers who use it every day.
As well as resurfacing the road, the project also includes the installation of replacement kerbs, a refurbished drainage system, new safety barriers, road markings and new reflective cat's eyes.
READ MORE: Norfolk A47 dualling schemes legal challenge dismissed
WHAT HAPPENS NEXT?
The final phase of the roadworks will begin on Monday, August 7.
National Highways said that will involve a phased reopening of all slip roads, with one lane of travel open in each direction on the carriageway.
A contraflow crossover point for traffic on the main carriageway will need to be installed so that can happen.
That has meant the Tuttles Interchange southbound entry slip road has been closed and will remain shut until 6am on Monday, August 7.
After that, the Tuttles Interchange southbound entry slip road and the Spooner Row southbound entry slip road will both remain open to traffic.
National Highways said it will then reopen other slip roads between Spooner Row and the Tuttles Interchange, but cannot give dates yet.
Once the work is completed, both lanes of the A11 will reopen and the 70mph speed limit will be restored.
ANALYSIS: A STRANGE EXCUSE
For those of us who drive out of Norfolk with any regularity, the long-running roadworks on the A11 have been enormously frustrating.
It has now been more than a year since the work to resurface a section of the road between Wymondham and Spooner Row first started.
And it has transpired that the £65m work, which was originally due to be completed in June, will continue until the end of the summer.
National Highways says the reason for the delay in finishing the work is the weather. They say last year's hot summer, followed by a wet winter, means it has taken longer than hoped.
It's a strange excuse for the delay. Fair enough, last summer did have some exceptionally hot spells. Record temperatures were recorded and it is understandable why that would prevent work happening so rapidly.
But, given Anglian Water was, just a couple of months ago, issuing warnings about drought conditions for East Anglia after a dry winter, that element of the excuse does sound jarring.
Clearly, this work needed to be done. The surface of the A11 made for a bumpy ride for many motorists and improved safety barriers are to be welcomed.
However, the impact this work - on one of the key routes in and out of Norfolk - cannot be overstated.
For people in Wymondham and Hethersett, it has brought the double-edged sword of making it difficult for them to get on to the A11, while also leading to motorists using their local roads to try to get around the roadworks.
And it is a shame that it proved impossible to get this work finished before the region's peak summer tourist season began. It's not a great introduction to a week or so in Norfolk to be stuck in a traffic jam on the way in to the county.
Council bosses have long been critical of National Highways, believing that Norfolk is at the bottom of their list of priorities.
Multi-million pound work on sections of the A47 is due to begin in the coming months, so all eyes will be on how smoothly that work goes.
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