Contracts that have already been agreed for work in the North Sea “should not just be ripped up”, the prime minister has said.
But Boris Johnson said there was a need to “transition as fast as we reasonably can” away from oil and gas.
The prime minister was asked about whether plans for fossil fuel extraction from the Cambo oil field near Shetland risked sending mixed messages ahead of the UK hosting the COP26 climate summit in Glasgow later this year.
“This was a contract that was agreed in 2001 and we can’t just tear up contracts. There’s a process to be gone through,” Mr Johnson said.
Asked during a broadcast pool clip if the drilling would be a “good look” ahead of the climate summit, the prime minister replied: “I think it’s a fantastic look for this country that we are the world-leader now in offshore wind power.”
He said North Sea oil had been a “huge part of the UK economy for decades now”, but added: “We recognise that and there has got to be a smooth and sensible transition.
“But that doesn’t mean there aren’t massive opportunities to increase the use of green technology.”
Pushed on if he would set a deadline for ending fossil fuel extraction, Mr Johnson said: “Look at what we’ve done already. We’ve transitioned away from coal in my lifetime.
“Thanks to Margaret Thatcher, who closed so many coal mines across the country, we had a big early start and we’re now moving rapidly away from coal altogether.”
The PM’s comment about Thatcher has drawn criticism from opposition parties.
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Cambo is an oil field containing an estimated 800 million barrels of oil, co-owned by Shell and the private equity backed Siccar Point Energy.
The license to develop the field was awarded in 2001, but UK government regulator The Oil and Gas Authority is due to give it final approval to go into production.
If the go-ahead is given, a further 150-170 million barrels are due to be drilled from the site, which would be expected to operate until 2050.
More than 80,000 people have signed a petition calling on Mr Johnson to stop the development and block any extraction of fossil fuels amid government pledges to cut carbon emissions.
Mr Johnson said he recognised that tackling climate change was “going to be a tough ambition, this is a difficult thing to achieve”.
But he called on fellow world leaders to meet the challenge, saying: “What we won’t do, we will not reduce the level of our ambition for COP, in order to set the target, an ambition that we know we can meet.
“I’m going to be as ambitious as possible for COP26 in Glasgow. I want the world to recognise the extent of the challenge, and I want everybody to try to rise to meet it in the way that I just set out with those ambitions.
“We must, must, must be as ambitious and as tough as possible and that’s what we’re going to do.”
The prime minister said phasing out coal power across the world by 2040 was “doable”.
“What we are saying to the whole world, as we come forward to COP in November, we want the whole world to move away from hydrocarbons.
“We are setting a deadline for the end of coal, we want everybody to give up coal by 2040, that’s one of the targets we are setting at the COP summit to happen in Glasgow.”
But Labour leader Sir Keir Starmer demanded action not “soundbites” from Mr Johnson on the issue.
“We’ve got a UK prime minister who bundles around with a cabaret of soundbites, with targets about climate change but doesn’t put in place the action,” he said.
“We all know that hydrogen and wind are part of the future, we haven’t got an industrial strategy, we haven’t got a hydrogen strategy.
“Get your head out of the sand, stop the soundbites, let’s have some action.”
Sky News has launched the first daily prime time news show dedicated to climate change.
The Daily Climate Show is broadcast at 6.30pm and 9.30pm Monday to Friday on Sky News, the Sky News website and app, on YouTube and Twitter.
Hosted by Anna Jones, it follows Sky News correspondents as they investigate how global warming is changing our landscape and how we all live our lives.
The show also highlights solutions to the crisis and how small changes can make a big difference.