The 44th president of the United States was seen entering through the black door of Number 10 Downing Street at around 3pm, and left around an hour later.

It is understood the pair spoke about a range of subjects, including artificial intelligence - mostly one-on-one.

Politics latest: Obama arrives in Downing Street for unannounced meeting

The former president also met with Labour leader Sir Keir Starmer.

Mr Obama left his post after serving two terms in the White House between 2009 and 2017 - the limit for a leader in the US.

He was in London as part of his work with his foundation, Sky News understands, and that he was there on a "courtesy" visit.

Ahead of the Brexit referendum in 2016, he announced the UK would be at the "back of the queue" in any trade deal with the US.

No deal has yet been signed between the UK government and Washington DC - although there have been agreements reached between the UK and some states like Texas and Utah.

Mr Obama visited Downing Street numerous times while president, meeting Gordon Brown and the now foreign secretary, Lord David Cameron.

He also met Theresa May while their tenures in office overlapped, although this happened at summits abroad rather than in Westminster.

A Downing Street spokesperson said the meeting was "informal", and that Mr Obama and Mr Sunak met in the study.

The spokesperson added: "I think President Obama's team made contact and obviously the prime minister was very happy to meet with him and discuss the work of the Obama Foundation."

According to the Obama Foundation's website, the organisation aims to "help people turn hope into action-to inspire, empower, and connect them to change their world".

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Mr Obama used the phrase "hope" as part of his election campaigning.

The foundation also says it's "supporting and connecting values-based changemakers around the globe, by working to create a world where girls have equal opportunities to pursue their dreams through access to education, and by fostering safe and supportive communities where young men of colour have clear pathways to opportunity".

The visit comes as Mr Sunak faces difficulty in domestic politics, with his polling numbers falling and reports that Tory MPs and even ministers want to dethrone him.