Image caption Arguments for and against the relief road have been ongoing for decades

A final decision on the M4 relief road should be taken by Carwyn Jones's successor, the assembly's environment committee chairman has said.

The first minister told AMs last month he would be final "decision-maker" on the new stretch of road around Newport.

But Labour AM Mike Hedges said it should be made by an incoming, rather than an outgoing, first minister.

A public inquiry's recommendation is due to be published by early autumn. Mr Jones is stepping down in December.

AMs have been told they will have a vote on the M4 but it is still unclear whether it will be binding on the Welsh Government.

Welsh ministers favour the so-called "black route" - a £1.4bn 15-mile stretch of new motorway to the south of Newport.

The public inquiry has also looked at other options, including cheaper alternatives.

Mr Jones told assembly members last month: "I don't think I've expressed a strong preference for either route, and nor can I, because I'll be the decision maker who takes the final decision."

Image copyright BBC/Google Image caption This map shows the route of the proposed M4 Relief Road - the "black route" is in green

But Swansea East AM Mr Hedges said: "You would expect a big decision to be made by the incoming, rather than an outgoing first minister.

"I think it would be beneficial as the new first minister would be responsible for it.

"It would be the first item on his or her desk. Otherwise you could have a situation in which the first minister puts it in, and the new first minister takes it out."

However, that is not a view shared by all.

Local Government Secretary Alun Davies said: "In my view the key to a smooth and orderly transfer is that we keep governing throughout this period.

"And this is what I expect Carwyn Jones to do. Anything else would erode his position and could prevent us from taking any difficult decisions until next January, which is not a good place to be."

Image caption A Welsh Labour leadership contest to find Carwyn Jones's successor begins in the autumn

AMs have been told there will be a vote on the plans later in the year.

Leader of the House Julie James, who organises government business in the assembly, said in February Welsh ministers were mindful to have a debate and vote in the Senedd.

She said that vote would not be binding on ministers, but added it was impossible to be sure about the nature of that vote until the public inquiry had been completed.

Economy Secretary Ken Skates has since said ministers were consulting lawyers on the form and the timing of the debate.

If the so-called "black route" is approved by the public inquiry, it is possible a legal challenge could cause further delays.

'Swift resolution'

Finance Secretary Mark Drakeford, the only declared candidate in the race to be the next first minister, has said a cheaper alternative to the M4 relief road "would be attractive".

Welsh Conservative leader Andrew RT Davies said the relief road "now looks dead in the water", calling on Mr Jones to go "sooner rather than later".

"Decisions of this magnitude can't just be parked until January and we need to see a swift resolution of the Labour leadership crisis," he said.

Analysis by BBC Wales political editor Nick Servini

Mike Hedges believes that not only is this the biggest financial decision remaining for Carwyn Jones, but could be the single largest item of public expenditure in Wales in his lifetime.

And he is not alone among Labour AMs who believe the successor to Carwyn Jones should not have their hands tied by inheriting such a decision made by a predecessor.

If there is a legal challenge to the public inquiry report then it is likely to push any government decision into the new year, and into the hands of the new first minister.

That will be welcomed by those who oppose the new stretch of motorway and believe their case is strengthened with someone other than Carwyn Jones in the top job.

The first minister has now given himself a quasi-judicial role as the final decision-maker saying he has not expressed an opinion on a preferred route but he has nevertheless led a government whose preferred option is the £1.4bn black route.

On that basis, the assumption is he will support the more expensive option if he is given the opportunity.