Image copyright @A465_Section2 Image caption The project is aimed at improving safety on the A465 and regenerating the region's economy

Wales' biggest road building project is costing taxpayers about £100m more than planned, a new report has said.

Transforming the A465 into a dual carriageway between Gilwern and Brynmawr was meant to be finished by September 2018 but is now pegged for completion in April 2021.

Auditor general Adrian Crompton said those living locally were paying a "higher than expected price".

The Welsh Government said the scheme was complex, but would bring benefits.

The five mile-long project in the Clydach Gorge has been hit by legal disputes between the Welsh Government and road builder Costain.

The geology of the area has been at the centre of the challenges and what has been found on site has been more difficult to build on than anticipated.

Both sides have argued over who should pay for the increase in cost.

The Wales Audit Office (WAO) report into the delays said it was impossible to say how long the dispute would take to resolve and the uncertainty meant it was difficult for the Welsh Government to predict the final figure.

It is part of a larger project to widen the A465 and was first envisaged in 1994.

In 2014, costs to the public purse were estimated by the Welsh Government at £223m, but this has risen to £321m.

However, the overall estimated project cost is much higher and is not included in the report due to commercial sensitivity. Costain believes the Welsh Government's estimated share is understated.

The WAO said the level of disruption to local communities had been greater than expected. Six weekend closures had initially been agreed for safety reasons, but by the end of November the section of A465 had been closed for 57 weekends.

Mr Crompton said: "This is not the first time that the Welsh Government has faced difficulties with significant cost increases and delays on road projects and it is vital that lessons are learnt for future infrastructure schemes."

Media playback is unsupported on your device Media caption Heads of the Valleys dualling "has been hell"

The scheme involves replacing the old three-lane road, with builders fitting the new road into the gorge with split carriageways - with one part on a different level to the other - utilising a number of retaining walls.

These have formed part of one of several complicated legal disputes as the parties argue how to deal with increased construction costs.

According to the report, elements of the design developed by Costain for the walls were considered by the firm to be "impossible and/or illegal to construct in compliance with relevant standards/regulations or have been changed for other reasons".

"The changed designs are costing significantly more to construct", the report said.

Arbitration led to the Welsh Government and Costain splitting the increase, but the two parties remain in dispute about who is liable for a significant portion of the increase in cost.

Image copyright A465 Section 2 Image caption Economy Minister Ken Skates says the road is more than 85% complete

Other issues have included asbestos found at a former factory site, where the Welsh Government had to foot the bill.

Legal fees have mounted - with the Welsh Government having spent £1.49m by last November.

Economy Minister Ken Skates said on Tuesday that builders have delayed the end date to April 2021.

"I am obviously very disappointed by this further delay and have asked the project team to continue to explore what can be done to bring forward these programmed dates," he added.

Mr Skates said a total of 65% of the project's cash has been spend with Welsh firms, with 270 new jobs created and 69 apprentices trained.

Helen Mary Jones, Plaid Cymru economy spokeswoman, said: "There are clearly serious issues with this project and for the sake of public interest, the Welsh Government must release all details so that real scrutiny can take place."

Welsh Conservative business spokesman Russell George said: "It is not first time that the Welsh Labour Government has managed roads projects that have suffered significant cost increases and delays, and its increasingly vital that lessons are learnt, and learnt now, for future infrastructure schemes."

The Welsh Government said the project was "widely recognised as one of the most complex road engineering projects in the UK at the moment".

It added: "We recognise the frustration caused by the issues highlighted in the report, and will of course consider its findings."