The UK government still has no plans to publish an explosive report showing the national security risk from the collapse of ecosystems around the world – even after handing the key official behind the report a new advisory role.

Andy Burnham addressed the Cop31 climate summit in Turkey on Thursday and said the UK was determined to reach net zero by 2050 and work with its partners to build a clean energy future.

The prime minister also announced the appointment of Madeleine Alessandri, a long-serving civil servant and the former head of the UK's joint intelligence committee, to be adviser on climate security. The JIC is the committee of the UK's spy chiefs, which oversees the work of MI5 and MI6.

Alessandri spearheaded the publication of the climate report, which found that food shortages, food price rises, mass migration and nuclear conflict overseas could result from the failure to prevent the collapse of key ecosystems, including the Amazon rainforest.

The report was supposed to be launched at last year's Cop, but was suppressed by Keir Starmer's government. The Guardian revealed – through a leak – the key contents of the report ahead of its intended launch last year, and early this year a redacted version was put out. Ed Miliband, the foreign secretary, has briefed foreign governments on the findings and the report clearly influenced Burnham's decision to appoint Alessandri, but the report itself remains under wraps.

Burnham told Cop31 that Britain was ready to play its full part. "The message I carry here, in my first Cop as prime minister, is that the UK remains determined to reach net zero by 2050 and to work with our partners to build a clean energy future," he said.

"The world has changed. The consensus we once enjoyed on science and climate action has broken down. We will not stand by, and that's why I've pledged a new era of British leadership over the next decade."

He said he embraced the focus at the climate summit on electrification, because "if we depend on energy we do not control, we depend on prices we cannot control. The shift to clean energy is the only route to energy security."

But he insisted that industry must do its part to help people deliver the green transition. He highlighted progress on electric cars and heat pumps, adding: "If you drive a new electric car today in the UK, it costs less than a petrol equivalent. Get on the right tariff and a heat pump can be cheaper to run than a gas boiler."

Burnham's public declaration that the climate crisis poses a major national security threat cheered Cop31 and will be noted by governments worried about the impact of global heating on food supplies, the cost of living, migration and infrastructure. He is also one of the few leaders from western Europe to address the summit.

But campaigners, experts and green businesses and investors are worried about his commitment to the climate crisis because his chancellor, John Healey, is known to view economic growth as taking priority even if it means locking the UK into expensive fossil fuels.

Louise Haigh, who is first secretary of state and a key ally of Burnham, is thought to take a similar view, along with Jonathan Reynolds, the business secretary.

Behind the scenes, these concerns are shared by senior members of his government. Significantly, Burnham has no dedicated climate adviser working for him in Downing Street – despite it being an important issue to many voters.

A series of major decisions that have climate implications are being taken largely in isolation, rather than with their cumulative impact in mind, senior insiders have told the Guardian.

While Burnham addressed Cop31, his ministers were thought to be putting the final touches to a weakening of the mandate requiring carmakers to switch an increasing proportion of their sales to electric vehicles.

Burnham has also greenlighted the Jackdaw gasfield, cut funding for heat pumps and is mulling backing a new oilfield, Rosebank. Healey, meanwhile, has told MPs on the Treasury committee that Labour backs a third runway at Heathrow.

Campaigners and business people told the Guardian that the prime minister gave the impression of being swayed by short-term economic thinking from advisers not up to speed with a vastly changed energy world.

Ami McCarthy, head of politics at Greenpeace UK, said of Burnham's speech to Cop: "[It] hit the right notes, but noises offstage about a massive new oilfield and looser rules on electric cars almost drowned him out.

"Appointing the world's first climate security envoy is a welcome move, but approving Rosebank would only increase the climate threat level to communities in Britain and around the world.

"There are so many elephants in the room, you'd think it's a zoo. Burnham needs to start bridging the widening chasm between words and deeds on climate action."

But the prime minister has influential supporters too, who believe he understands the climate crisis and supports efforts to tackle it.

Burnham missed out on a bilateral meeting at Cop31 with Luiz Inácio Lula da Silva, the Brazilian president whose resounding re-election has given renewed hope to leftwing parties fighting the far right.

But he did manage a meeting with Fatih Birol, the executive director of the International Energy Agency and the world's foremost energy economist.

In between football talk – Burnham is famously an Everton fan, and Birol follows the Premier League closely – they discussed how cheaper energy from renewable sources, and more electrification of the UK economy, could bring down the cost of living.

Birol told the Guardian: "I can tell you that his mind and heart are fully behind the electrification of the UK energy system. I can put my name behind it. I had a very good and in-depth discussion with him."

The government was contacted for comment.