GREEN MP Caroline Lucas broke ranks with Welsh members yesterday by calling for a Plaid Cymru vote in seats where her party is not standing.

Ms Lucas has worked closely with Plaid Cymru and SNP MPs as part of a joint parliamentary group since being elected in 2010.

Now she has made a personal appeal for voters to support Plaid’s Hywel Williams, praising his “strong commitment to social and environmental justice.”

Ms Lucas added: “Just as Plaid Cymru have urged voters in England to back the Green Party, if there is no Green candidate standing, I would urge Welsh voters to back candidates like Hywel Williams.”

Her endorsement comes after Plaid peer and general election chief Dafydd Wigley urged 400,000 Welsh ex-pats living in England to vote Green on May 7.

Her statement is a major boost for Mr Williams, who is facing a tough fight with Labour’s Alun Pugh to retain the marginal Arfon seat.

Arfon is one of five seats in Wales which the Greens are not contesting.

But Ms Lucas risked angering Welsh Green leader Pippa Bartolotti, who has refused to form an electoral pact with Plaid Cymru.

Talks between Ms Bartolotti and Plaid leader Leanne Wood broke down in February, with the Wales Green leader complaining of “social media harassment” by nationalist supporters.

Mr Williams stressed the two parties shared positions on cuts, fracking and Trident nuclear missiles as he welcomed Mr Lucas’ endorsement.

He said: “For the last five years we have acted very much as a group and this has been a great success — securing debates on austerity, opposing Labour and Tory cuts.”