A ban on selling new petrol, diesel or hybrid cars in the UK will be brought forward from 2040 to 2035 at the latest, under government plans.
The change comes after experts said 2040 would be too late if the UK wants to achieve its target of emitting virtually zero carbon by 2050.
Boris Johnson unveiled the policy as part of a launch event for a United Nations climate summit in November.
He said 2020 would be a "defining year of climate action" for the planet.
The summit, known as COP26, is being hosted in Glasgow. It is an annual UN-led gathering set up to assess progress on tackling climate change.
Sir David Attenborough said at the launch event at London's Science Museum that he was looking forward to COP26 and found it "encouraging" that the UK government was launching a "year of climate action".
"The longer we leave it... the worse it is going to get," he said.
"So now is the moment. It is up to us to organise the nations of the world to do something about it."
In a statement made ahead of the launch, Mr Johnson said the ban on selling new petrol and diesel cars would come even earlier than 2035, if possible.
Hybrid vehicles are also now being included in the proposals, which were originally announced in July 2017.
People will only be able to buy electric or hydrogen cars and vans, once the ban comes into effect.
The change in plans, which will be subject to a consultation, comes after experts warned the previous target date of 2040 would still leave old conventional cars on the roads following the clean-up date of 2050.
The Scottish government does not have the power to ban new petrol and diesel cars but has already pledged to "phase out the need" for them by 2032 with measures such as an expansion of the charging network for electric cars.
Mr Johnson said the 2050 pledge was necessary because the UK's "historic emissions" meant "we have a responsibility to our planet to lead in this way".
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