Proposed changes to the A303 at Stonehenge would have an "adverse impact" on the "outstanding universal value" of the site, Unesco's World Heritage Committee has said.
At a meeting in Baku earlier, it confirmed its position as unchanged from its draft decision.
Highways England finalised plans for a two-mile (3.2km) dual carriageway tunnel in January 2017.
But the committee urged the UK government not to allow the upgrade.
The busy A303 currently passes within a few hundred metres of the ancient monument in Wiltshire. Highways England has said a tunnel will cut congestion and improve journey times.
The committee acknowledged improvements to the scheme but it said the route upgrade should not proceed "in its current form".
Instead, it proposed longer tunnel sections, "which reduce further the impact on the cultural landscape".
The tunnel, part of a £1.7bn upgrade of the Wiltshire road, aims to remove the "sight and sound of the road" as it passes the stone circle.