A £1bn relief road for the M4 around Newport has been confirmed by Welsh Transport Minister Edwina Hart.
It is the largest capital investment programme ever announced by the Welsh government.
Supporters say it will improve transport in south Wales, ease congestion and boost jobs.
But opponents have criticised the plan because of its environmental impact and one group has dubbed it a "billion pound mistake."
The new road will run between Junctions 23 and 29 and include a new bridge and could be completed by the spring of 2022.
Three routes were under consideration, but Mrs Hart told AMs on Wednesday that "none of the alternative solutions proposed could reasonably deliver the objectives" of easing congestion.
She said it was important that Wales has "an effective economic infrastructure which improves our competitiveness and access to jobs and services."
Environmental and some business groups have criticised the preferred route as unnecessarily damaging and costly.
Friends of the Earth Cymru said the decision will see a six-lane motorway "plough through part of one of Wales' most protected and environmentally sensitive landscapes" - the Gwent Levels.
Director Gareth Clubb said: "This vastly expensive, utterly unnecessary and terribly damaging motorway is the worst possible transport choice for south east Wales.
"And the £1bn price tag means that, at a time when traffic has actually decreased since 2007, it's going to come at the cost of the sustainable solutions that could benefit us all.
"This Welsh government seems stuck in a past age of concreting over our beautiful countryside and committing our economy to a fossil fuel future we won't be able to afford."
Transport group Sustrans Cymru said the move was a "wrong turn" when there was a need for more public transport.
The announcement brought mixed reaction from the business sector, with CBI Wales welcoming the decision while FSB Wales, which represents small businesses, calling it a "billion-pound mistake".
CBI Wales Director Emma Watkins said her organisation had consistently judged the preferred route, known as the black route, as the most prudent and effective option.
"The M4 around Newport is simply not fit for the 21st Century. That stretch of motorway is Wales' main economic artery, it is already congested and is predicted to get worse," she said.
"I agree with the prime minister when he said it was a 'foot on the windpipe of the Welsh economy'."
However, FSB Wales spokesman Iestyn Davies said: "It is deeply disappointing that the minister has turned her back on the blue route proposed ...a scheme which could provide an effective solution to the current problems on the M4 a full decade earlier than the scheme she is progressing and for around £600m less."
Shadow Transport Minister Byron Davies gave the news a qualified welcome, saying "the spades must start digging and businesses and travellers given the relief so sorely needed".
But he insisted "under no circumstances must that come at the expense of scrutiny - and today's last-minute announcement has failed to take environmental concerns into account, or provide relevant detail".
Plaid Cymru and the Liberal Democrats favour a cheaper, alternative relief road route, involving upgrading the A48 Newport Southern Distributor Road.