Joe Biden and King Charles have held a meeting on how to tackle the climate emergency, after the US president earlier discussed Ukraine’s counteroffensive with Rishi Sunak in Downing Street. Biden, on a brief stopover in the UK before the Nato summit in Vilnius, Lithuania, hailed the “rock-solid” friendship between the UK and the US. The president did not attend the king’s coronation in May, sending his wife, Jill, in his place. The visit to Windsor Castle for tea was said to be a reflection of the US president’s appreciation of the seriousness with which King Charles took the climate crisis. Along with his climate envoy, John Kerry, and the UK energy secretary, Grant Shapps, Biden met executives and philanthropists for a private discussion on climate finance for the world’s poorest countries. The king, an environmentalist for more than 50 years, launched the sustainable markets initiative in 2020 at Davos, bringing together global CEOs to raise climate finance. He has also tried to make the Commonwealth a market leader in addressing the issue. The two men have joined forces on green campaigns before, with TV cameras picking up Biden telling the then Prince of Wales at the UN Cop26 climate summit that “we need you badly” in the global drive to lower emissions. At Windsor Castle, Biden was given a royal salute and a rendition of the US national anthem courtesy of the Welsh Guards. The meeting with Sunak, their fifth in as many months, was dominated by discussion of Ukraine and its aspirations for Nato membership, a subject that is likely to dominate the Nato summit and has the potential to reveal frictions in the UK-US alliance. Before the start of formal discussions in the Downing Street garden with Sunak, Biden said: “We’ve got a lot to talk about. Our relationship is rock solid. Couldn’t be meeting with a closer friend and a greater ally.” No 10 said Sunak wanted to find a pathway for Ukraine to join Nato. Kyiv had said the previous US requirement that Ukraine first fulfilled a Nato membership action plan had been dropped, something that is regarded as a breakthrough by the Ukrainian foreign ministry, even if some preconditions will apply. Although the Downing Street transcription of the hour-long meeting made no mention of the membership action plan, Ukraine’s foreign secretary, Dmytro Kuleba, said the precondition had been removed. In his talks with Biden, Sunak focused on the planned “security guarantees” an informal alliance of Nato countries intended to provide Ukraine as an interim measure before it received the full protections of Nato membership. The long-term guarantees first proposed by the UK are intended as a sign to Vladimir Putin that western resolve to support Ukraine will not waver and that he has no option but to come to the negotiating table, or else his political authority would erode further. Biden and Sunak also agreed to differ over the US decision to accede to Ukraine’s request to provide cluster munitions on certain conditions, and as a bridge until the supply of more conventional armaments picks up. The UK prime minister’s official spokesperson said providing the weapons “was a difficult choice for the US” that had been “forced on them by Russia’s war of aggression”. The two leaders “discussed the commitments that UK has under that convention, both not to produce or use cluster munitions and to discourage their use”. Asked if Sunak complied with that commitment to discourage the use of the weapons in his talks with Biden, the spokesperson said: “Yes, they discussed the requirements the prime minister is under because of this convention, and the UK is upholding that.”