National Grid officials have been accused of failing to provide enough information about a £100 million tunnel planned under the Menai Strait.

A full meeting of the Isle of Anglesey County Council unanimously passed a motion calling on the utility company to meet with the authority and provide further information about its plans for transporting the electricity generated by the proposed Wylfa Newydd nuclear plant.

To avoid building pylons near the Menai Strait, National Grid is proposing a 4km tunnel under the Strait, despite calls for collaborative working with the Welsh Government so that a proposed third Menai crossing could incorporate the cables and scrap the need for a tunnel altogether.

The company says it intends to forge ahead with the tunnel as its customer, Horizon Nuclear Power, requires a connection by the mid-2020s when Wylfa Newydd is expected to be operational.

Cllr Carwyn Jones, portfolio holder for major developments at Anglesey Council, said: “We’ve been hitting our heads against a brick wall for years. The people of Anglesey are being treated with contempt.

“National Grid officers haven’t been up here to speak to us for three years.

“The newer members haven’t had a chance to ask any questions at all. That simply isn’t good enough.”

Gareth Williams, National Grid’s senior project manager for the North Wales Connection project, has refuted the accusations, claiming that more than 40,000 pages of technical documents have been shared and 120 meetings have taken place with council representatives to discuss the plans.

He said: “These have included 80 individual workshops on the detail of our proposals and how we are trying to mitigate their effects – including how we intend to build the tunnel under the Menai Strait and detailed information on traffic and transport.

“The council’s leadership and officers have replied with over 300 pages of formal responses.”

The tunnel is set to be buried up to 100 metres deep and to measure five metres in diameter, with tunnel-boring machines drilling under the surface and lining it with concrete.

Proposed temporary road closures in the area surrounding Llanfairpwll could be in place for several years in order to accommodate heavy goods vehicles as the work goes on.

According to Cllr Jones, however, the vast majority of local residents are not aware of the scale of the project, which could be submitted for approval by the Planning Inspectorate in late summer. “The proposed tunnel will mean around a million tonnes of rock and sludge being brought to the surface, but where will all of it go?” he said.

The council, as well as the island’s MP and AM, have also been steadfast in their opposition to a new row of overhead pylons to transport Wylfa Newydd’s output across Anglesey.

Cllr Jones said: “How can we plan and mitigate for such a massive development when so little information is forthcoming from the National Grid?

"We’re not getting any answers. The Grid’s only plan is the cheapest option: pylons all the way from Wylfa Newydd. They’ve made up their minds but we’re putting on additional pressure.

"We know there’s been delays in Horizon’s application for a development consent order (DCO) for Wylfa Newydd, so this downtime is a chance for the Grid to properly consult and also look at other options including sharing a third Menai crossing.”

But Mr Williams said: “We have also consulted extensively with local communities and have received hundreds of pieces of feedback from them on all aspects of our project.

“We note the motion due to be discussed by the council and would stress that we follow all relevant planning legislation and guidance when planning and carrying out work in Wales.”

A separate motion was also passed, calling on the Home Secretary to look at strengthening the 2008 Planning Act and to bring it in line with the Welsh Well-being of Future Generations Act (2015), which would ensure that adequate mitigation was deliverable.