Greta Thunberg has told the BBC she does not plan to attend the UN climate conference due to be held in Glasgow this November.
The 18-year-old Swedish climate campaigner is concerned about the impact of the coronavirus pandemic on attendance at COP26.
She believes the summit should be postponed.
She says the UK government, which is hosting the summit, should wait until global vaccination rates have risen.
The summit will bring together world leaders with the aim of agreeing a plan to tackle climate change.
Greta’s decision is likely to be a significant blow for the UK government.
The activist has attended every major climate conference since her first protest outside the Swedish parliament two and a half years ago.
She said: “This needs to happen in the right way. Of course, the the best thing to do would be to get everyone vaccinated as soon as possible so that everyone could take part on the same terms.”
The UN meeting has already been delayed once, from November 2019, and there have been rumours that it may be postponed again.
The last two Conference of the Parties (COP) summits have had more than 20,000 attendees and the UK is understood to have been working on the basis that as many as 30,000 people could attend in Glasgow.
At the end of last month, sources in Downing Street and Holyrood were adamant that no decision had been made on a further delay to the conference.
However, the BBC was told a final decision on whether to delay or move ahead with the summit was likely to be taken shortly after Easter.
Greta Thunberg would not play a formal role at the conference but her decision not to attend is a significant symbolic moment for the conference.
Since she began her first school strike alone outside the Swedish parliament two and a half years ago she has come to symbolise the demands of many young people that world leaders take climate change more seriously.
“How dare you”, she told world leaders at a UN climate summit in New York in September 2019, as she accused them of failing to act on climate change.
“You have stolen my dreams and my childhood with your empty words”, she told them.
Speaking in a new documentary series “Greta Thunberg: A Year to Change the World” , she was clear what she thought the COP26 team should do.
She told the BBC she thought the summit should be postponed again.
Mark Carney, the former Bank of England governor who is now Finance Advisor for COP26, would not be drawn on whether he thought the conference would happen in person when asked in an interview for the BBC podcast, Newscast.
“I think if we’ve learned anything in the last 13 months, it’s that’s it’s not wise to make bold predictions about what’s going to happen three, six months ahead in terms of logistics, so the honest answer is I don’t know”, Mr Carney told presenter Adam Fleming.
Any decision on the future of the summit would be made in conjunction with the United Nations, as well as the Scottish government.
At the end of last it month it was reported that Prime Minister Boris Johnson was battling to ensure the flagship conference was held face-to-face by ensuring participants were vaccinated and subjected to a robust testing regime.
But the global rollout of vaccinations has been disappointing.
Last year, the decision to delay the meeting – because of Covid – was taken in May.
Greta Thunberg: A Year to Change the World begins on BBC One at 21:00 on 12 April and will be available on the BBC iPlayer