The £1bn M4 relief road around Newport has been approved by Welsh Transport Minister Edwina Hart.
It is the largest capital investment project ever announced by the Welsh government.
The new road will run between Junctions 23 and 29, with a new bridge, and could be completed by the spring of 2022.
However, the scheme has received a mixed reception, with supporters saying it will improve transport links in south Wales, and opponents criticising the route chosen because of its environmental impact, and its cost.
The announcement follows a consultation, with three routes under consideration. The ‘black route’, the most southerly of the three, has been selected by Hart.
But there was anger that a cheaper fourth option, involving upgrading the A48 Newport Southern Distributor Road, was not considered. The Welsh Assembly’s cross-party environment committee expresses “grave concerns” over the issue.
Labour minister Hart said 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.”
Conservative 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 had favoured the discounted A48 Newport Southern Distributor Road option.
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.”
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”.