Boris Johnson has announced a “world-leading” new legal target to cut the UK’s greenhouse gas emissions by nearly four-fifths by 2035.

The Prime Minister described the target to reduce emissions by 78% on 1990 levels by 2035, which will be set in law under the Government’s sixth carbon budget, as the most ambitious in the world.

It has been announced on the eve of a major US summit on Thursday where President Joe Biden is expected to set out a new US target for reducing emissions.

And it comes as the UK Government seeks to show climate leadership ahead of the United Nations Cop26 summit in Glasgow in November, aimed at driving stronger action by countries to address the crisis.

The new target is in line with advice from the Government’s advisory Committee on Climate Change, whose chief executive Chris Stark welcomed it as an historic decision that puts the UK at the forefront of global climate efforts.

But campaigners and opposition politicians warned the UK is off-track to meet existing targets and have called for urgent policies and investment to deliver emissions cuts.

Environmentalists also say it makes the Government’s £27 billion road-building scheme and planned airport expansions even harder to justify.

The 2035 target is part of efforts to drive down greenhouse gases to “net zero” by 2050, with emissions cut as much as possible and any remaining pollution “offset” by measures that absorb carbon such as planting trees.

In its advice on the target last year, the Climate Change Committee set out widespread changes across the economy and society that would be needed to meet the goal.

They include switching from petrol and diesel cars to electric vehicles and from gas boilers to clean home heating, more offshore wind power, a reduction in meat and dairy consumption and planting of new woodlands.

But the Government said it was not taking on each of the committee’s policy recommendations, and said it would meet the target through “new green technologies and innovation, whilst maintaining people’s freedom of choice, including on their diet”.

Announcing the move, Mr Johnson said: “We want to continue to raise the bar on tackling climate change, and that’s why we’re setting the most ambitious target to cut emissions in the world.

“The UK will be home to pioneering businesses, new technologies and green innovation as we make progress to net zero emissions, laying the foundations for decades of economic growth in a way that creates thousands of jobs.

“We want to see world leaders follow our lead and match our ambition in the run-up to the crucial climate summit Cop26, as we will only build back greener and protect our planet if we come together to take action.”

It means that every choice we make from now must be the right one for our climate

Mr Stark said: “In committing to cut emissions by almost 80% in 2035, the UK has taken its place at the forefront of global efforts to reach net zero – crucial in the fight against climate change.

“Now we have set this goal in law, Government, business and people up and down the country can throw their full weight behind the actions needed to get us there.”

And he urged: “It means that every choice we make from now must be the right one for our climate.”

For the first time, the target will also include the UK’s contribution to international shipping and flights, which the Government said would support the sectors to reduce emissions and develop clean technologies.

And it builds on a pledge made last year to cut emissions by 68% by 2030 as part of commitments under the global Paris climate accord.

But shadow business secretary Ed Miliband said the Government had repeatedly failed to match ambitious promises on emissions with effective action on the ground.

“While any strengthening of our targets is the right thing to do, the Government can’t be trusted to match rhetoric with reality,” he said.

“This year, as hosts of Cop26, the UK has a particular responsibility to lead the world and show the way forward for a greener future. This Government isn’t up to the task.”

Friends of the Earth climate campaigner Connor Schwartz warned the UK government was already struggling to meet its existing, less ambitious climate goals.

“Boris Johnson’s Government may want to show global leadership ahead of this year’s crucial climate summit, but with its backing for new roads and airport expansion, and huge financial support for a mega gas development in Mozambique, it won’t be taken seriously,” he warned.

Greenpeace UK’s head of politics, Rebecca Newsom, said the announcement could be the boldest pledge in the week of the US summit, which is expected to see a number of countries increase their ambition on climate action.

“This major shift in gear from the Government makes destructive projects like new road-building and airport expansion even harder to justify.”

And she said: “In order to actually deliver on this commitment, new measures to slash emissions from homes and transport should already be well under way.

“So unless the Government’s policies urgently fall in line with its ambitions, there could still be awkward questions for Boris Johnson at the global climate talks in the autumn.”

The Government has indicated it will set out a net zero strategy for how it will cut emissions to meet the 2050 goal ahead of the talks in November.