Image caption Mark Drakeford will decide on Tuesday whether to build the M4 relief road

The long-awaited decision on whether the £1.4bn M4 relief road will be built will be announced later.

If it goes ahead it will be most expensive infrastructure project the Welsh Government has ever embarked on.

But commentators expect First Minister Mark Drakeford to make a U-turn and ditch plans for the road south of Newport.

Secretary of State for Wales Alun Cairns said this would have a "major negative" impact on the economy.

"All we need is a positive decision," he told Claire Summers on BBC Radio Wales.

It follows a year-long public inquiry into proposals supported by the business group the CBI but opposed by environmentalists.

Since the UK government announced borrowing powers for the Welsh Government, ministers have pursued building the new six-lane motorway, re-opening proposals dating back to 1991.

Welsh Labour's manifesto for the 2016 assembly election - under the leadership of Carwyn Jones - said the party would deliver "an M4 relief road".

Mr Cairns added it would be five years or more before the benefits of the road would be seen, if it did go ahead.

Media playback is unsupported on your device Media caption What is it like living next to the M4?

Mr Jones was succeeded by Mr Drakeford last year and the Cardiff West AM, who has been considering the results of the public inquiry which will also be published Tuesday, is thought to be more sceptical.

If he does scrap the scheme, it will be the third time since the beginning of devolution Welsh ministers have shelved the project.

The plans for the M4 relief road were described as "government-sponsored ecocide" at the public inquiry.

But dozens of businesses called for the government to press ahead, with the CBI's Leighton Jenkins said the relief road will deliver £2 for every £1 invested.

Image copyright Getty Images Image caption Environmentalists have long-opposed the plans for the new motorway

Haf Elgar, director of Friends of the Earth Cymru, said: "The M4 relief road would be a disaster for the wildlife on the Gwent Levels, and lead to more air pollution and climate-wrecking emissions being pumped into our atmosphere.

"With the world facing a climate emergency and our natural environment in crisis it's unimaginable that the Welsh Government would go ahead with this damaging new road."

But Monmouthshire council leader Peter Fox tweeted his support for the plans, saying a toll road would be better than no road.

Ray Adams, whose business Niche IFA overlooks the M4, said he supported the plans.

"I live in Newport and work in Newport and I don't want to be stuck in traffic myself as well. So I'd like to see the road go ahead."

Media playback is unsupported on your device Media caption M4: What could you spend £1.4bn on?

The plans have proved politically divisive too - there has been significant opposition in Labour and some ministers are thought to oppose the scheme.

Plaid Cymru is firmly against, while the newly-formed Brexit Party is backing the road.

Some Labour Newport politicians - particularly those from the Newport West constituency - have lobbied for the road to be built.

Labour AM Jayne Bryant said: "Air pollution is a serious public heath issue because of the congestion on the M4.

"The vast amount of traffic on the M4 around Newport is not local to Newport - that's why public transport alone will not solve it."

Plans for the scheme were revived in 2013 after the UK government offered borrowing powers that would allow for the upgrade.

Former Prime Minister David Cameron called the Brynglas tunnels "a foot on the windpipe of the Welsh economy" - and the UK government remains a supporter of the scheme.

Media playback is unsupported on your device Media caption The proposed route of the M4 relief road

Analysis - Felicity Evans, BBC Wales political editor

We do not know for sure what Mark Drakeford will announce, but the consensus is he will say no to the "black route" - the six-lane motorway across the Gwent Levels that was the subject of the planning inquiry.

He may well announce measures to mitigate the congestion problems at the Brynglas tunnels and possibly other infrastructure plans - partly because there is a discomfort at the highest levels of Welsh Government that really big infrastructure projects never seem to get the go-ahead in Wales, either from the Welsh or the UK governments.

A claim Wales "is a nation of artists' impressions" has really stung with some ministers.