Former U.S. President Barack Obama called in at the British Prime Minister’s official residence on Monday, but the office is keeping tight-lipped about the purpose of the one-hour meeting.

Barack Obama surprised when he turned up at Rishi Sunak’s front door on Monday, going inside the historic Downing Street residence for an hour and meeting with the Prime Minister. Former President Obama walked up to the famous black door alone but later emerged alongside the U.S. Ambassador to London Jane Hartley.

Former US President Barack Obama waves as as he arrives outside 10 Downing Street in central London, on March 18, 2024, ahead of his meeting with Britain’s Prime Minister Rishi Sunak. (Photo by Adrian DENNIS / AFP) (Photo by ADRIAN DENNIS/AFP via Getty Images)

Former US president Barack Obama arrives for a meeting in Downing Street, London. Picture date: Monday March 18, 2024. See PA story POLITICS Obama. Photo credit should read: Stefan Rousseau/PA Wire (Photo by Stefan Rousseau/PA Images via Getty Images)

LONDON, ENGLAND – MARCH 18: Former United States President Barack Obama gestures as he and Jane Hartley (yellow jacket), the U.S Ambassador to the United Kingdom, leave 10 Downing Street after meeting UK Prime Minister, Rishi Sunak, on March 18, 2024 in London, England. President Obama has been in Europe this week and appeared at a moderated debate “An Evening with President Barack Obama” last night in Antwerp. The evening focussed on his vision of the future and the challenges the world is facing. (Photo by Carl Court/Getty Images)

While Obama and Sunak clearly had enough to be talking about, the reason for the visit and what was discussed was not immediately disclosed, the mystery talk being described by an official to British government-adjacent newspaper The Daily Telegraph as “informal” and private. The Times notes shortly before Obama entered Downing Street, the British finance minister also arrived at the Prime Minister’s official office and residence by the front door. That arrival may be seen as a pointed statement given it is thought to be possible to get to the Prime Minister’s office from the Chancellor’s through internal passageways without going onto the street to be seen by the press.

Barack Obama’s visits to Downing Street may not have the happiest associations for some: his meetings there in 2016 ended in him telling the British people not to vote for Brexit, and if they did the country would go “to the back of the queue” for a trade deal with the U.S., which was seen as a potential benefit of leaving the European Union. This hectoring position, of telling sovereign people how to vote was not perceived to have been well received at the time, and the UK subsequently voted against Obama’s wishes.

While President Obama was out of the White House shortly after, he threat was made actual by eventual successor Joe Biden, who has stalled on trade with the UK since, leaving the country seeking deals with individual states instead.

LONDON, ENGLAND – APRIL 22 2016: US President Barack Obama (L) and British Prime Minister David Cameron during a press conference at the Foreign and Commonwealth Office on April 22, 2016 in London, England. The President and his wife are currently on a brief visit to the UK where they attended lunch with HM Queen Elizabeth II at Windsor Castle and later will have dinner with Prince William and his wife Catherine, Duchess of Cambridge at Kensington Palace. Mr Obama visited 10 Downing Street on this Friday afternoon and held a joint press conference with British Prime Minister David Cameron where he stated his case for the UK to remain inside the European Union. (Photo by Ben Stanstall – WPA Pool/Getty Images)