Most new cars must be electric by 2030, ministers told
Three-fifths of new cars must be electric by 2030 to meet greenhouse gas targets, ministers have been warned.
Homes also need to be built to a higher standard, the Committee on Climate Change - the official watchdog - says.
The government says the UK is cutting emissions faster than any other G7 nation - and the committee agrees there has been a big shift under Theresa May.
However, it says the UK will fall short of its ambitions unless ministers do more to turn pledges into reality.
The warning comes less than a week after the prime minister launched a 25-year plan to protect the environment, including eradicating all avoidable plastic waste by 2042.
The committee agrees the government's recently-published Clean Growth Plan is a big improvement, and says the UK has been a world leader in cutting emissions so far.
But it argues that the plan still doesn't offer detailed policies to meet legal carbon targets.
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Carbon capture from industry must be made to happen, it says, and wood and plastics should be banned from landfill in order to re-use them.
More trees should be planted to soak up carbon dioxide, with a view to creating 70,000 hectares (173,000 acres) of new woodland by 2025, and farming must do more to cut emissions.
Industry, too, is urged to take greater responsibility.
The committee's chairman Lord Deben, told BBC News: "If you're going to sell an electric car your dealers have got to understand these things, so training dealers is essential.
"If you're running a big fossil fuel company, you have to start thinking about the realities of when, not if, because it is not if any longer, we use a lot less fossil fuels."
He also criticised construction firms for only doing the "absolute minimum" required on building energy efficient homes.
The committee points out that better insulated homes would cut people's bills as well as tackle climate change, and calls for more incentives to encourage "able to pay" households to install efficiency measures.
Lord Deben said the Clean Growth Strategy had "changed the tone" of the government on the issue.
"These issues have been put into the centre of government policy - that's a major change."
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But, he said, ambitions alone are not enough.
"The strategy doesn't deliver enough action to meet emissions targets in the 2020s and 2030s," he said.
"The government's policies will need to be firmed up as a matter of urgency and supplemented with additional measures if the UK is to deliver on legal commitments and secure its position as an international climate change leader."
He added: "All departments now need to look at their contribution towards cutting emissions - including the Department for Transport."
The committee wants 30% to 70% of new cars to be ultra-low emission by 2030, as well as up to 40% of new vans, as part of efforts to phase out sales of conventional petrol and diesel versions by 2040.
Currently, fewer than 5% of new car sales are "alternatively fuelled", which also includes hybrid models.
Prof Michael Grubb, from University College London, said: "There are plenty of good ideas out there on low-carbon energy, cutting emissions from buildings, clean transport and more, but as the committee rightly points out, concrete plans need to be put in place, and soon.
"The government is making all the right noises on support for the low-carbon economy, but these must be turned into action: we need a year of decision-making."
Richard Black, from the Energy and Climate Intelligence Unit, agreed: "We're not on track to meet emissions goals that kick in in just five years' time.
"That leaves ministers little time for enquiries and consultations - they're going to have to put new policies in place fast."
Mr Black suggested quick-win policies including: cutting company car tax for electric vehicles; repealing the ban on onshore wind power (the cheapest form of electricity generation), and re-starting the programme for Zero Carbon Homes.
A business department spokesperson said: "The scrutiny of the independent Committee on Climate Change plays an important role.
"The UK has reduced emissions on a per-person basis faster than any other G7 nation, and our clean growth strategy is the next ambitious milestone in our work to de-carbonise the UK.
"We have always said it is only the start of a process.
"Our proposals will continue to evolve - whether in response to costs of renewable energy coming down, improved evidence about climate change, wider trends in technology or the economic opportunities delivered through our industrial strategy."
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