Boris Johnson has travelled to Ukraine to meet President Volodymy Zelensky as questions mount over his financial affairs at home.
The ex-Prime Minister visited Bucha and Borodyanka to the north of Kyiv on Sunday morning, the scenes of some of the worst atrocities committed by Russian forces.
Footage showed Mr Johnson meeting the Ukrainian president, warmly shaking hands and telling him "it's an absolute honour to be here again".
"Honour for us," Mr Zelensky replied.
His spokesman confirmed he had been invited to visit by the Ukrainian President.
Mr Johnson is believed to regard the UK's backing for Ukraine as one of his key legacies after pushing international allies to offer support at the beginning of the war.
He is a popular figure in Ukraine and he struck up a friendship with President Zelensky, with critics sometimes accusing him of using calls to Kyiv as a distraction when he faced political troubles at home.
His visit comes as Labour called on Parliament's sleaze watchdog to investigate claims that the BBC chairman helped Mr Johnson guarantee a loan of up to £800,000 - weeks before he was recommended for the job by the then-PM.
His surprise trip is likely to raise eyebrows in London as it could be seen as undermining Rishi Sunak's authority.
No10 insisted that Mr Sunak is "supportive" of Boris Johnson's visit to Ukraine.
The Prime Minister's press secretary said he is "always supportive of all colleagues showing that the UK is behind Ukraine and will continue to support them".
Mr Johnson said: "It is a privilege to visit Ukraine at the invitation of President Zelensky.
"The suffering of the people of Ukraine has gone on for too long. The only way to end this war is for Ukraine to win – and to win as fast as possible.
"This is the moment to double down, and to give the Ukrainians all the tools they need to finish the job.
"The sooner Putin fails, the better for Ukraine and for the whole world."
As a former prime minister, Mr Johnson's security protection for overseas trips is funded by UK taxpayers.
Asked who is stumping up for his trip, his spokesman said they could not get into specifics "for security reasons".
The spokesman insisted he "fully supports" Government policy on Ukraine.
Last week, Mr Johnson compared Vladimir Putin to "the fat boy in Dickens" who wants to "make our flesh creep" with threats of using nuclear weapons.
Speaking at a breakfast event at the World Economic Forum in Davos, he said: "Putin wants to present it as a nuclear stand-off between between Nato and Russia.
"Nonsense. He's not going to use nuclear weapons, okay. He's like the fat boy in Dickens, he wants to make our flesh creep. He wants us to think about it. He's never going to do it."
