THE M4 relief road may not be complete until 2023 – two years later than planned – and its costs have increased by about 10 per cent to £1.3 billion, it has been revealed.

Revamped plans also include a massive new bridge at Newport Docks which, at 146 metres at its highest point, will be nine metres taller than the second Severn crossing and twice as large as the Newport Transporter Bridge.

Concessions to Association of British Ports (APB), which runs Newport Docks, have meant the costs of the proposed black route – which will run through the south of the city, bypassing the congested Brynglas Tunnels but cutting straight through the docks – will cost an extra £136 million, taking the scheme’s overall predicted price tag to £1.3 billion.

The 440 metre-long bridge will run between the south and north docks. And the Welsh Government has said some extra quay space will be built at the south docks to accommodate larger ships which may not be able to get into the north dock as a result of the bridge.

The revamped plans have reportedly been developed alongside APB.

Newport Docks is the busiest port in Wales, supporting 3,000 jobs and generating £186 million a year.

The next cheapest alternative, building the new road north of the docks, would reportedly cost £100 million more.

A public inquiry into the plans began earlier this year and will resume next month.

In a statement the Welsh Government’s economy and transport secretary Ken Skates said he hoped the new plans would allow APB, which had previously objected to the scheme, to withdraw its objections.

“A resilient port industry plays a vital role in our economy and society, and it is of course right that the potential new interface of road, rail and maritime infrastructure is well planned with a long-term view,” he said.

He added: “It is still expected that works could commence as soon as next summer.

“I must stress that the works would only be constructed, and funding allocated for this purpose, if a decision is made to build the M4 corridor around Newport project.

“I anticipate that final decision can be made in the summer in light of all views being heard at the public inquiry and the inspectors’ report being received.”

An ABP spokesman said: "The port of Newport plays a vital role supporting the Welsh economy.

"That's why it's important to safeguard the future of the port, protecting jobs and trade.

"We welcome the Welsh Government's commitment to achieving that objective."

The Welsh Conservatives' shadow economy secretary Russell George called the news of the cost increase and delay "very disappointing".

“Congestion is tightening its grip on Welsh roads, and yet there doesn’t seem to be a single Labour transport project running on time and within budget," he said.

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“As a result, Welsh taxpayers face a bill of almost £200 million to cover the cost of the delays – money which could have been better spent on other schemes all over the country.

“It’s disastrous news for the Welsh economy, and for motorists in the region who have faced endless delays to this vital upgrade of the M4.”

And Plaid Cymru MP Jonathan Edwards called the announcement "sensational".

"The Labour government has misled the people of Wales about the costs of the M4 black route," he said.

"In 2014 Labour promised that it would cost less than a billion pounds, and that it was a quick solution to the issue of congestion.

"Three years later, and costs have spiralled, and years of delays have been added.

"A trail of deceit exists around this scheme whereby the impact on business has been exaggerated and traffic projections inflated."

He added: "Plaid Cymru will be opposing the M4 black route when it comes to the relevant vote in the National Assembly.

"We urge opposition parties, independents and Labour backbenchers who have opposed this scheme to join us when the time comes."

Gwent Wildlife Trust’s Chief Executive Ian Rappel said: “As the Welsh Government’s Public Inquiry into the proposed new M4 is on-going, we are disappointed that the Minister Ken Skates has been making statements about the motorway plans, before the Inquiry has concluded and the Inspector’s recommendations have been made.

“Welsh Government’s latest announcement is indicative of the spiralling costs to the taxpayer, as a result of this fallacy.

“Their suggested new plans change nothing and Gwent Wildlife Trust remain vehemently opposed to a motorway being built across Wales’ equivalent of the Amazon rainforest.

“The proposed new M4 is massively damaging, totally unsustainable and uneconomical for both people and wildlife and its route will destroy an irreplaceable and precious area of the Gwent Levels forever.”