The independent report for the county council said A12 improvements are essential if Sizewell C goes ahead.
Traffic will increase on the stretch through Stratford St Andrew, Marlesford, Farnham and Little Glemham.
EDF Energy, planning to build Sizewell C, said it was not considering funding for the bypass as a priority.
The county council report looked at the impact of construction traffic on the A12 and predicted it would be slightly reduced compared to earlier studies.
However, the additional number of vehicles generated by projected "natural" growth, in addition to the Sizewell C project traffic, meant road improvements were essential.
The report recommends a dual or single carriageway bypass should be built and ready before work starts on the new power station.
The report said that traffic at the busiest time of the day could increase by 14% and the proportion of vehicles taking more than seven minutes to travel through the four villages would more than double.
The high levels of Sizewell C traffic would result in increased journey times, a higher accident rate and poorer air quality, the report said.
Tom McGarry, from EDF Energy, said it was listening to local concerns but its priority was getting cars off the road in the first place.
Guy McGregor, Suffolk County Council's cabinet member for roads and transport, said: "Two months ago we said that EDF Energy needed to wake up and truly realise the scale of the impact Sizewell C will have on local communities in Suffolk.
"We hope that this independent report from experts with international repute will help them come to their senses now.
"It's time for EDF to take note of the strength of feeling and undeniable evidence on this critical issue."
"The A12 from Woodbridge to Lowestoft is just not fit for purpose - let alone to carry the volumes of heavy lorries in particular which will be necessary for the Sizewell C project," a Suffolk Coastal council spokesman said.

Belle Gibson gained fame in Australia after she claimed to have beaten cancer using natural remedies rather than medical intervention.
She launched a successful app and cookbook, but later admitted the diagnosis was made up.
Consumer Affairs Victoria is aiming to prosecute Ms Gibson for allegedly breaking Australian consumer law.
The regulator said it had conducted an in-depth investigation of Ms Gibson's activities and had applied to Australia's Federal Court for leave to pursue legal action.
Ms Gibson's claims to have cured her cancer with Ayurvedic medicine, oxygen therapy and a gluten and refined sugar-free diet propelled her to prominence.
She capitalised on her fame with a successful app and a cookbook, both called The Whole Pantry, and promised to deliver a share of the profits to several charities.
But the money allegedly never reached the charities and cracks began to appear in Ms Gibson's story.
She later admitted to the Australian Women's Weekly magazine that none of her claims was true.
Attempting to justify her behaviour, she told the magazine: "If I don't have an answer, then I will sort of theorise it myself and come up with one. I think that's an easy thing to often revert to if you don't know what the answer is."
Ms Gibson's publisher, Penguin Australia, has already agreed to pay A$30,000 ($22,200; Â£15,300) to the Victorian Consumer Law Fund as a penalty for releasing The Whole Pantry, which was not fact checked.
Four villages in Suffolk must be bypassed to take anticipated growth in traffic due to the construction of a nuclear power station, a report says.
[NEXT_CONCEPT]
An Australian blogger and author who faked terminal brain cancer faces legal action over her deception.