Landscape charity Friends of the Lake District said the approval of the A66 scheme in Cumbria is short-sighted and will harm the environment.
The scheme will see the remaining single carriageway sections along the 50-mile stretch of the A66 between Cumbria, County Durham and North Yorkshire dualled.
But Friends of the Lake District said the new dual carriageway sections road would harm the landscape and tranquillity of the North Pennines AONB, damage internationally important peat bogs and significantly increase carbon emissions at the time Cumbria is working to reach net zero by 2037.
It also questioned how it aligned with an aim to reduce car journeys and increase sustainable travel in Cumbria and claimed it was contrary to the new Levelling Up and Regeneration Act 2023 as it does not further the purposes of statutory bodies, the Lake District National Park Authority and the North Pennines AONB.
The charity said safety upgrades should be made to the road instead of dualling and challenged the argument that the A66 was the most dangerous road in the UK.
Dr Kate Willshaw, policy officer, said: “At a time when so many of us are experiencing the impact of climate change, it is morally wrong to embark on a project that will lock-in greater car use. What we need is action to rapidly cut emissions and encourage massive behaviour change about car use.
“In addition, Parliament has just passed new legislation which means that bodies such as National Highways must seek to further the purposes of national parks and AONBs.
“This is much stronger than previous legislation and means that any development that affects national parks and AONBs must actually include measures which don’t just mitigate harm, but actually make the environment of the protected landscapes better.
“Upgrading the A66 will in fact do the opposite and damage both the Lake District and the North Pennines AONB.
“We are bitterly disappointed by this short-sighted decision by the Secretary of State and we will be considering our next steps.”
