Image copyright Thinkstock Image caption The Britannia Bridge carries the A55 and a railway line between the mainland and Anglesey

Construction of a third crossing over the Menai Strait could begin by 2021 if it gets the go ahead, the Welsh Government has said.

Consultants are to be hired later this year to look at routes for a proposed new crossing to Anglesey.

Funding options to pay for the scheme are also to be examined.

Plaid Cymru welcomed progress on the proposal, saying current traffic congestion in the area causes "unnecessary misery" for commuters.

Currently, there are two Menai Strait crossings: the Menai Bridge, which was built by Thomas Telford in the 1820s, and Robert Stephenson's Britannia Bridge, which was opened to carry rail traffic in 1850 but rebuilt in the 20th century to also carry road traffic.

The A55 runs as a dual carriageway from Holyhead to the Britannia Bridge, where it is carried as a single carriageway road, before returning to a dual carriageway on the other side.

Image caption Gwynedd council leader Dyfed Edwards says Wylfa Newydd would make a third bridge necessary

There have been proposals for a third Menai crossing for some time to alleviate traffic on the two bridges over the strait.

According to a Welsh Government document, Economy Secretary Ken Skates has now "agreed to appoint technical advisors to take forward the third crossing project".

He also decided to appoint "external support to advise on possible financial packages to fund the project".

Dyfed Edwards, Plaid Cymru council leader for Gwynedd council, said "it is absolutely key that there's a new crossing" if the new Wylfa Newydd nuclear plant development "comes into place".

"It's not just commuter traffic, it will be construction traffic as well," he said.

A Plaid Cymru spokeswoman said: "This is a project which we have long championed yet which is sadly still to materialise due to the Labour Welsh Government's lethargy on the matter."

She said: "Current traffic congestion causes unnecessary misery for commuters on a daily basis and there are already concerns over the existing two bridges' long-term resilience."

Russell George, the Welsh Conservatives' economy spokesman, said the project was "long overdue".

"The third bridge has been on the agenda for more than a decade", he said, referring to a consultation in 2007.

"Yet the Welsh Government have done nothing," he added.

"The time for announcements is over - action is what is now needed."

Image copyright PA Image caption The Menai Bridge was built in the early 19th Century

A Welsh Government spokesman said: "We are currently in the process of procuring consultants to carry out a route selection study for the third Menai crossing.

"This will appraise various options and designs for the third crossing and will include a public consultation.

"Construction could start in 2021 subject to gaining all the necessary statutory consents."

Consultants are expected to be hired in the autumn, with the study to take 18 months.