The former US president and ex-prime minister have known each other for many years, working together when leading their respective countries.
And while they have previously been aligned on policy issues, like Brexit, they have also diverged in recent times – most notably on Ukraine.
So what are their respective histories with the country and the conflict?
Both men have had involvement with Ukraine in the last few years – but under rather different circumstances.
Boris Johnson was prime minister when Russia invaded the country last February, and was a leader in helping rally international support for the Ukrainian people and military.
President Volodymyr Zelenskyy considers him a friend, having spoken very warmly of him, and Mr Johnson continues to lobby the international community to provide aid and weapons even now he has left office.
Meanwhile, Mr Trump has something of a chequered history involving Ukraine, dating back to before Russia’s invasion last year.
In September 2019, reports emerged that he had asked President Zelenskyy to dig up dirt on the then-former vice president, Joe Biden, who was expected to run against him in the 2020 presidential election.
That phone call led to Trump’s first impeachment on charges of abuse of power and obstruction of congress – but he was not convicted after a trial in the Senate.
When it comes to the war, Trump has refused to say who he thinks should prevail, telling a town hall broadcast (part of his 2024 presidential campaign) earlier this month that he’d end the war in just 24 hours.
He said: “I want everybody to stop dying. They’re dying. Russians and Ukrainians. I want them to stop dying. And I’ll have that done in 24 hours.”
