The Senedd has narrowly rejected calls for all new power lines to be placed underground.

Adam Price led a debate on a Plaid Cymru motion to make it mandatory for all new electricity distribution lines to be underground rather than overhead pylons.

He explained that current Welsh Government policy says new power lines should be laid underground but it includes a caveat on cost grounds.

Mr Price warned the policy is not working as intended, saying: “As long as the caveat exists, developers will always exploit it and build pylons as their preferred option.”

He said this has been a catalyst for a “rash of proposals for long-range pylon lines traversing large swathes of our country”, including Carmarthenshire and Ceredigion.

He urged the Welsh Government to mandate underground power lines, following the example of other European countries, saying underground cables do not spoil the landscape, are cheaper to maintain and more reliable.

Mr Price welcomed a review but said: “If we want to prevent the kind of mass pylonisation that much of our country is currently facing, then we can't afford to wait for the outcome.”

Russell George, MS for Montgomeryshire, raised concerns about overproliferation of windfarms and power lines “thundering” through the hills of Mid Wales.

Siân Gwenllian, the Plaid Cymru MS for Arfon, warned the caveat allows costs to take precedence over environmental, social and aesthetic factors.

Julie James, who is responsible for planning, said the Welsh Government and opposition are not miles apart but she took issue with the “problematic” wording of the motion.

The former lawyer acknowledged that the Welsh Government needs to “tighten up what we mean by ‘unaffordable’ in a very big way”.

Ms James said Jeremy Miles, who is responsible for energy, has set up an independent advisory group and Planning Policy Wales will be updated to reflect its review.

Cefin Campbell, who represents Mid and West Wales, warned Wales’ beautiful landscape is being “sacrificed on the altar of profit”.

“We must underground these cables,” he said. “In doing so, we as a Senedd will be taking a strong stance to protect the natural beauty and the ecology of our unique landscapes.”

With the vote tied 25-25, David Rees – the Senedd’s deputy speaker or Dirprwy Lywydd – broke the deadlock by using his casting vote against the motion.

Plaid Cymru, the Conservatives and Jane Dodds, the Lib Dems’ leader in Wales, backed the motion, while Labour voted against.