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.
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.