Building a bypass around Stonehenge instead of a tunnel underneath would have less impact on the landscape, a Unesco report says.
The report scrutinised the routes considered but ruled out by Highways England before the public consultation began for the current tunnel option.
More people and experts should be included in the consultation, it added.
Unesco's report goes against three conservation bodies, including the National Trust, who want a tunnel.
Highways England says it will listen to all views.
A Highways England spokesman said it was investing £1.4bn to improve the A303 near Stonehenge, and would "continue to actively engage and listen to everyone who has an interest in this scheme".
'Pollution and delays'
The report added that the two tunnel options would cause "considerable damage...through adverse effects on the archaeological remains, on their landscape attributes, and on setting and visibility".
The best option was route F010, from Berwick Down then between Berwick St James, and Winterbourne Stoke, it said.
It would then continue east, south of Stonehenge and south of Boscombe Airfield before picking up the A303 east of Amesbury.
The option had been excluded, because of its affordability, cost and length, according to the Highways England report.
A spokesman for Historic England, National Trust and English Heritage said: "We're disappointed that the ICOMOS report largely ignores both the benefits of removing a large stretch of the A303 and the danger of doing nothing at all."
The report also recommended an "open forum" as not enough consideration was given to the views of archaeology experts and the wider public.
President of Salisbury Chamber of Commerce and Industry, Andy Rhind-Tutt, agreed with the report findings.
He added the bypass would provide better access to main roads in the area, improve congestion in Salisbury with minimal impact on the nearby villages.
"The revised route would not cost anywhere near the build or running costs of a tunnel," he added.
Wiltshire Council has said it would consider Unesco's report and advice.