Peter Hain confessed he would oppose Britain’s official departure from the European Union in the likelihood he is given a vote in the House of Lords.
The former minister of Europe said although he acknowledged the historic referendum result, quitting the bloc would damage Welsh jobs.
The Government is expected to lose a Supreme Court ruling this week – which would require the activation of Article 50, the formal method by which the UK leaves the EU, is put to a vote in both Houses of Parliament.
Peter Hain said he would vote against triggering Article 50 in the House of Lords
I can’t in all conscience and principle vote for that
Lord Peter Hain
If that is the case, Lord Hain said he would vote against the will of the British people, claiming Brexit would be damaging to the poorest communities.
Speaking on BBC News, he said: “I recognise the referendum result and respect it.
“But what we were not promised, as the prime minister has now done, is a Brexit deal that is extremely right wing, damaging to Welsh jobs and damaging to our most vulnerable people.
“I can’t in all conscience and principle vote for that.”
Lord Hain had campaigned for Britain to Remain within the EU
Lord Hain’s admission puts him at odds with Jeremy Corbyn, who told fellow politicians to accept the prime minister’s plan to leave the single market.
The Labour leader suggested his party could impose a three-line whip on the Article 50 vote, meaning any MPs or peers who voted against the plans would be rebelling.
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He made the comments amid reports some of his own shadow cabinet ministers were thinking of voting against the mechanism for getting the formal process for Britain’s EU divorce started.
Jeremy Corbyn urged fellow Labour politicians to back Theresa May's plans
In an interview with the Guardian, Lord Hain said Mr Corbyn could relate to his desire not to back the party line on this issue.
“I’m sure Jeremy Corbyn understands because he voted with his conscience disobeying the whip hundreds of times,” he continued.
“I’ve never done that – I’ve had a very occasional rebellion. But I cannot bring myself to vote for something that is going to be so damaging for Britain.
“We are heading for the precipice and we are about to jump over and I can’t support.”
The comments made by the former Secretary of State for Wales come as the country’s First Minister demanded Britain remained inside the single market.
Despite Wales voting to Leave the EU, Labour’s Carwyn Jones insisted a vote for Leave didn’t have to mean exiting the Brussel’s trading bloc.
As he prepared to unveil a joint Brexit plan for the country along with Plaid Cymru leader Leanne Wood, Mr Jones said he was instead in favour of Norway’s approach, which allows full access to the single market but enables free movement for those who have jobs.
“This enables us to respect the result of the referendum. We are not here to re-fight that," he told the Today programme on Monday.
"We've offered a common sense approach on immigration, where we say there's a freedom of movement to work.
"What we wouldn't say is that there's a general freedom of movement without restriction."