Andy Burnham has said the UK has “lost more than we gained” since leaving the EU, the prime minister’s latest in a string of comments criticising Brexit.

Outlining his desire to form closer ties with Brussels, Burnham accused the Conservative party of having “burned all our bridges” with the bloc after Brexit and said a new deal he hopes to secure with the EU next week will “get food prices down” and put more money in the pockets of British exporters.

The prime minister is due to meet European leaders at a summit next week and secure agreements that he says could raise GDP by 0.4% – about £17bn – including on food and farming, emissions trading and a youth mobility scheme. He will also “explore options” for a different long-term relationship with Brussels.

Speaking at the Welsh Labour conference on Saturday, Burnham said: “We need to be willing to take difficult decisions to get the cost of living down.

“It’s why, in a few days’ time, I will head to the next UK-EU summit and I will be working hard as we move towards it to get to a new deal on food and farming between the UK and the EU. Why am I doing that? Because I want to help Welsh businesses.

The prime minister added: “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.”

Burnham has recently said he is open to rejoining the single market, customs union or to “go all the way” by rejoining the EU, but on Saturday he said voters would ultimately be “the boss” of any changes.

He reiterated his commitment to Labour’s manifesto “red lines” – which rule out returning to the customs union, single market or free movement. “People working together in unions, trade unions, but also unions of nations, and that’s what makes us who we are,” he told the conference.

“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.”

The government has already made several announcements that would tie the UK closer to Brussels. Earlier this month, the Guardian reported that the UK was prepared to spend up to £450m a year to give EU students discounted university tuition fees, if Brussels allowed Britain access to the Made in Europe industrial programme.

Earlier this week, Hamish Falconer, the cabinet minister in charge of EU talks, said Britain was willing to discuss paying towards infrastructure schemes in poorer parts of the EU in return for access to the European electricity market.

Despite Burnham’s recent pro-EU comments, senior figures in Brussels have said they want to see the UK begin to start paying for access to the European schemes it says it wants access to.

The EU’s former Brexit negotiator Michel Barnier told the BBC last week: “There will be no possibility to have one freedom and not the other. The four freedoms are clearly the four conditions to be inside a single market.”

Ursula von der Leyen, the president of the European Commission, and António Costa, who leads the European Council, will meet Burnham on 20 November in Brussels for a summit where deals on food and drink, carbon emissions and youth exchange could be signed.

Barnier said the UK could not rejoin the single market “à la carte”, when speaking to the BBC’s Laura Kuenssberg last Sunday, adding: “I said very, very often during these negotiations, the Brexit, there is no cherrypicking.”