By Michael Holden
LONDON (Reuters) -U.S. President Joe Biden was due to arrive in Britain on Sunday evening for a brief visit during which he will meet Prime Minister Rishi Sunak and discuss climate change with King Charles ahead of the upcoming NATO summit in Lithuania.
The White House said the trip was designed "to further strengthen the close relationship between our nations".
The president will travel to Downing Street on Monday to hold a low-key meeting with Sunak, their fifth in as many months. Sunak's spokesperson said their discussions would likely include the upcoming NATO summit and Ukraine.
"As we face new and unprecedented challenges to our physical and economic security, our alliances are more important than ever," Sunak said in a statement released by his office on Saturday.
"The UK is Europe's leading NATO ally, we are the United States' most important trade, defence and diplomatic partner, and we are at the forefront of providing Ukraine with the support they need to succeed on the battlefield," said Sunak, who studied at California's Stanford University and owns a penthouse flat in Santa Monica.
Sunak has gone some way in repairing ties with Biden after the relationship cooled under his predecessors Boris Johnson and Liz Truss due to their tough stance over a post-Brexit deal with the European Union and Johnson's closer ties to Donald Trump.
For Biden, the more high-profile part of the trip will be his meeting with King Charles at Windsor Castle to the west of London, where the monarch's late mother Queen Elizabeth hosted Barack Obama in 2016 and Trump in 2018.
The king will receive Biden in the quadrangle of the castle, where a guard of honour will give a Royal Salute and the U.S. national anthem will be played, the king's office said.
The president and the king are due to discuss climate issues, a subject on which Charles, 74, has campaigned and spoken out about for more than five decades.
When the two men met at the COP26 U.N. climate summit in Scotland two years ago, Biden praised Charles' leadership on the subject, telling him "We need you badly".
"You are very kind for saying that," Charles replied.
Biden met Charles at a reception during a visit for the funeral of Queen Elizabeth last year, but in keeping with longstanding practice of U.S. presidents he did not attend the king's coronation in May.
U.S. special climate envoy John Kerry told the BBC he had been invited to brief the king and Biden about a climate finance conference that he was due to host with British energy minister Grant Shapps on Monday.
Following the meeting, Biden and Sunak leave Britain for Lithuania where NATO leaders will gather for a key summit. Biden is then expected to travel to Helsinki for a meeting with Nordic leaders.
(Reporting by Michael HoldenAdditional reporting by Muvija M and Paul SandleEditing by Helen Popper)