The King has welcomed world leaders, business figures, environmentalists and NGOs to Buckingham Palace for a reception ahead of the UN climate summit Cop27.
Charles will miss the gathering of world figures in Egypt next week but is likely to discuss issues around Cop27 and climate change with those invited to his royal residence.
Rishi Sunak was among a small group of guests who met the King in a separate room before the main reception was held in the ballroom and the Prime Minister gave a short address to those invited.
In the state dining room were leading figures including Labour leader Sir Keir Starmer and Governor of the Bank of England Andrew Bailey.
Earlier this week Mr Sunak reversed his decision to skip Cop27, bowing to pressure from activists, his own environment adviser and Boris Johnson.
Despite No 10’s U-turn, the King is still not planning to attend Cop27.
Downing Street acknowledged on Thursday that the King might have been able to join delegates in Egypt if the Prime Minister had been in office earlier in the year.
When Liz Truss was premier, a “unanimous agreement” was reached that the King should not attend after advice was sought by his office from the Government.
A Downing Street spokeswoman said on Thursday it was now not “logistically feasible” for the King to join the international gathering at Egypt’s Red Sea resort of Sharm El-Sheikh.
Among the guests were Sir Pascal Soriot, chief executive of AstraZeneca, former New York mayor and business news mogul Mike Bloomberg, and yachtswoman Dame Ellen MacArthur, whose foundation is working to tackle climate change.
In his speech the Prime Minister also paid tribute to the King for “…helping to find practical solutions to climate change and biodiversity loss for more than 50 years, long before Cop1 let alone Cop27.”
He added: “The reduction in global emissions pledged during our presidency is now equivalent to more than six times the annual emissions of the entire global aviation industry.”
Addressing the guests Mr Sunak said perhaps the greatest breakthrough of the UK’s presidency of Cop26 was an idea suggested by Charles – bringing together leading investors and businesses at Glasgow to direct the world’s wealth at protecting the climate.
He said: “As a result of what many of you here are doing today, we’re turning the fight against climate change into a fantastic source of new green jobs and growth.”
Charles ended the day by planting a lime tree in Buckingham Palace’s garden as part of the Queen’s Green Canopy project – thought to be the first time he has carried out a tree-planting ceremony as monarch.
The initiative has planted more than a million trees across the UK to honour and celebrate the Queen’s 70 years of service to the nation.