Approving the Rosebank oil and gas field in the North Atlantic would be a “death sentence”, climate activists have said.

Regulators are considering granting the Norwegian company Equinor permission to begin exploiting one of the largest reserves of fossil fuels in the region.

It would see up to 500 million barrels of oil pulled out over its lifetime with many scientists and campaigners voicing their opposition to the project in recent weeks.

Speaking at the Southbank Centre’s Planet Summer festival, the group of young activists that included Greta Thunberg and Mya-Rose Craig, said development of the oil field would exacerbate the destructive effects of climate change such as heatwaves and wildfires experienced in many parts of the world this summer.

Climate activist Tori Tsui said: “There are fires raging across the world, this is the hottest summer ever recorded.

“All the while our energy security and net zero Minister Grant Shapps says that we need to max out our oil and gas reserves. It is a death sentence.”

The Government and Offshore Energies UK, which represents UK oil and gas companies, have said extracting fossil fuels in the North Atlantic and North Sea is more environmentally friendly than buying it from other countries because it would save emissions on shipping.

Climate change minister Graham Stuart has repeatedly expressed his support for continuing to exploit the North Sea’s “declining basin” in preference to buying oil and gas from other countries.

Critics, including shadow climate change secretary Ed Miliband, said up to 80% of Rosebank’s oil would not be sold to the UK.

The North Sea Transition Authority has this month delayed its decision on whether to approve Rosebank, reportedly because of concerns that its operations emissions may not be compatible with the Government’s net zero target.

On Friday, the group of activists protested outside the office of Mr Shapps, urging ministers to block its development.

Ms Thunberg, who attended the demonstration, told a packed-out audience in the Royal Festival Hall it was her 258th week of protesting.

When asked how to make sure phasing out fossil fuels is on the agenda at the upcoming climate change conference Cop28, she said: “Raise our voices. I mean, that’s all we can do.

“We’re not the ones in power. The ones in the world with the most powerful voices, with the most resources, are the ones who are destroying the world.”

Ati Viviam Villafaña, from the Arhuaco people of Sierra Nevada de Santa Marta in northern Colombia, said that if we put all our hopes in stopping climate change at Cop28 we are “wasting our time”.

The UK Government, two years after hosting the UN climate conference in Glasgow, has begun reviewing its environmental policies after the narrow Conservative by-election win in Uxbridge and South Ruislip, where the Ultra Low Emission Zone (Ulez) was considered a deciding factor.

Prime Minister Rishi Sunak criticised Labour leader Sir Keir Starmer for changing his policies on the basis of recent headlines after reading headlines about Ulez being an electoral asset for the Conservatives.

Ms Craig, who has been an activist since she was 11 and founded a charity at 13, said she has been writing to Jacob Rees-Mogg, her local MP, about environmental issues.

She told the Southbank audience: “I’ve been writing letters to him. I think he must hate me. I’ll say, I saw you voted about this, we’re not happy about it.

“Because I think we need to remind our Government that this is an issue that is make or break for a lot of people.”

The hour-and-a-half talk finished with the group unveiling the same pink StopRosebank banner shown at their central London protest on Friday.