Friends of the Earth Cymru have confirmed today that a letter will be sent to the Welsh Government, asking them to withdraw and restart an environmental consultation about the proposed M4 relief road route.
The group say they are concerned about the environmental impact of the proposals, and claim alternative routes to the one through the Gwent Levels - which was announced as the preferred road earlier this month - were not given enough consideration.
The Welsh Government has two weeks to respond to the letter, and depending on the outcome of that, Friends of the Earth may decide to then begin judicial review proceedings.
Ministers have previously said they expected to face a legal challenge.
More top news
Windy overnight, with sleet and rain moving quickly east through the evening, with snow falling over the hills.
Plans for a new Velindre Cancer Centre have been approved by Cardiff Council
RSPCA Cymru is appealing for information after a French bulldog was found hairless and with a serious prolapse.