Andy Burnham accused the Tories of having “burned all of our bridges” in the aftermath of Brexit and said the country had “lost more than we’ve gained over the last 10 years”.

The prime minister’s remarks in recent weeks have fuelled questions about the prospect of the UK returning to the EU or adopting its freedom of movement rules, but Mr Burnham has insisted voters would ultimately be “the boss” of any changes.

At the 2016 EU referendum, 854,572 (52.5 per cent) voters in Wales chose to leave the EU, compared with 772,347 (47.5 per cent) supporting Remain.

Mr Burnham appeared at Welsh Labour conference in Llandudno as he acknowledged the party needs to “listen” and “take stock” following a major defeat in May’s Senedd election.

Defending his efforts to secure new deals with the EU, the Prime Minister told Welsh Labour conference: “We’ve got the Conservatives who are standing out against it, saying no change. Well, I don’t believe Brexit and the deal that they got has been good enough for Britain.

“I think we’ve lost more than we’ve gained over the last 10 years, and, actually, I think the Conservatives gave Britain a harsher and harder Brexit than people thought they were getting when they voted in 2016, because they burned all of our bridges in the aftermath, with the EU, and so I’m going to lead as Prime Minister on this issue, and I’m going to go and try and get the best deal I can for Wales, for those here who could benefit, Welsh farmers, exporters of Welsh shellfish and Welsh lamb.”

Andy Burnham said working with the EU would mean “respecting the red lines in our manifesto and respecting the referendum result”.

He told the audience: “People working together in unions, trade unions, but also unions of nations, and that’s what makes us who we are.

“And, yes, working with the European Union, as I said, but at the same time respecting the red lines in our manifesto and respecting the referendum, the referendum result. So, that’s the spirit, everybody, that I will take forward.

“Working with Wales as equal partners, and I know there is something that I can do to make a real difference.”

The prime minister is due to meet EU leaders in Brussels next week and sign agreements that he says could raise GDP by 0.4 per cent – including on food and farming, emissions trading and a youth mobility scheme.

In an interview this week, Lord Cameron, who resigned from Downing Street in the wake of the UK vote to leave the EU, said that starting a conversation about rejoining the bloc would be “very long and painful”.

Lord Cameron told Bloomberg he was “desperately disappointed” when the UK voted to leave the EU 10 years ago, but he warned against attempting to reverse the decision.

“Instead of launching an argument about rejoining, which I think would be a very long and painful argument – and who knows where it would end up – instead of that, focus on how do we be the best friend, best neighbour, best partner,” he said.

He said “flirting with rejoining” could hamper efforts to get a better deal, because EU countries might feel there is no point if the UK rejoins anyway.

“If you’re not going to, you know, commit to trying to make the partnership stronger because you’re flirting with rejoining, I think there’s some dangers there,” he said.