Boris Johnson will call on his G7 colleagues to do more to combat mass extinctions as he seeks to strengthen his environmental credentials.

Speaking in Biarritz, as discussion turns to the Amazon rainforest fires, the prime minister will call for more ambitious targets on biodiversity alongside measures to tackle the climate crisis, and reinforce the UK’s bid to host the COP26 climate change summit, due to be held next year.

“In a week where we have all watched, horrified, as the Amazon rainforest burns before our eyes, we cannot escape the reality of the damage we are inflicting on the natural world,” he will say.

Johnson will also announce that the UK will set aside an extra £10m of aid money to fund anti-deforestation projects in Brazil.

Labour accused Johnson of a “PR stunt”. The shadow trade secretary, Barry Gardiner, said: “The truth is that £10m is an embarrassingly tiny contribution to deal with the situation in the Amazon, which is part of the sustained anti-environment campaign being waged by a rightwing Brazilian government.

“Boris Johnson’s government has pointedly refused to ensure that UK companies are not aiding and abetting the destruction of the Amazon by ruling out from any trade deal beef raised on rainforest land burnt out by ranchers.”

G7 leaders will discuss the crisis in the Amazon, alongside other environmental issues, on Monday.

France and Ireland have said they will not sign the EU-Mercosur deal while the Amazon is burning. Asked en route to Biarritz whether he would echo their approach, Johnson said: “I think I would be reluctant to do anything at this very difficult time for global free trade, to cancel another trade deal.”

Theresa May made measures on global heating one of the themes of the final few weeks of her premiership, legislating for a new target of achieving zero net emissions by 2050.

But the issue of protecting biodiversity has also become increasingly salient in recent months, aided by the Extinction Rebellion campaign. Johnson will say the climate crisis and biodiversity are “two sides of the same coin”.

“The current global biodiversity targets have failed to stop the catastrophic decline in species. As the world’s largest economies we owe it to all nations and to future generations to do better,” he will say.

“We cannot sit back as animals and plants are wiped off the face of the planet by mankind’s recklessness. If we do not act now our children and grandchildren will never know a world with the Great Barrier Reef, the Sumatran tiger or the black rhino.

Johnson will also underline the UK’s calls for 30% of the world’s oceans to be protected by 2030.

However, the government’s commitment to the environment was questioned after weekend reports that No 10 was drawing up plans to cut fuel duty. The manifesto-friendly measure would be aimed at winning over motorists, and has long been advocated by campaigning MPs, including the Tory Robert Halfon.

But green groups cautioned that encouraging car use would be a backward step.

Aaron Kiely, a climate campaigner at Friends of the Earth, said: “It doesn’t add up for the government to commit to global biodiversity and pay the usual lip-service to the climate emergency while cutting fuel duty at home. Nothing short of radical changes are needed right now and getting off our oil addiction is long overdue. All the more reason why effectively encouraging more driving – probably to keep a segment of voters happy – shows a fundamental lack of seriousness.

“Funding to protect the Amazon is not to be sniffed at but better yet, we could listen to the indigenous people who live there and know perfectly well what is needed to prevent a worsening crisis: leave their land alone. Climate change is a problem everywhere and cohesive policy that applies at home as well as elsewhere is needed from this government.”

The transport secretary, Grant Shapps, also appeared wary of the idea of a fuel duty cut when asked about the policy on Sky News on Sunday. He said it was a matter for the chancellor, Sajid Javid, before talking up plans to increase the use of electric cars.