Boris Johnson has made a surprise visit to Ukraine amid renewed scrutiny over his personal finances.
The former prime minister said it was a "privilege" to be invited to the war-torn nation by President Volodymyr Zelenskyy, with whom he had a close working relationship during his time in office.
Downing Street indicated Rishi Sunak was "supportive" of the visit, after claims it could undermine his authority on foreign policy.
Mr Johnson was pictured visiting Borodianka near Kyiv - a town heavily damaged by the Russian invasion.
In a statement, Mr Johnson said: "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."
A spokesperson for Mr Johnson added that he fully supports UK Government policy on Ukraine, including the recent decision to send Challenger 2 tanks.
The ex-prime minister pitched himself as a key ally of Kyiv during his time in Number 10, providing support and calling on Western allies to follow suit in the early days of Russia's invasion last February.
As his scandal-plagued premiership unravelled, Mr Johnson was accused of using trips to Ukraine or phone calls with Mr Zelenskyy as a distraction for crises at home.
This trip comes amid allegations BBC chairman Richard Sharp helped the former prime minister arrange a guarantee for a loan - and that Mr Johnson later recommended Mr Sharp for the role of BBC chair.
Mr Johnson's spokesperson denied the report as "rubbish", but Labour has called for independent investigations to establish the facts behind the "murky affair".
The Guardian reported on Mr Johnson's plans to travel to Ukraine earlier in January.
Senior Tories raised concerns about the trip, with Commons defence select committee chair Tobias Ellwood telling the newspaper that Mr Johnson should "not interfere with the messaging or the official lines of communication" between London and Kyiv.
But Mr Sunak's press secretary said he is "always supportive of all colleagues showing that the UK is behind Ukraine and will continue to support them".
The prime minister has followed in the footsteps of Mr Johnson and Liz Truss in being a vocal supporter of Kyiv.
He visited Ukraine in November, pledging continued British support to the war-torn country in the fight against Moscow's forces.
Earlier this week, Mr Johnson spoke in a panel discussion on Ukraine at the World Economic Forum in Davos, where he was introduced as a "legendary figure" in Ukraine.
Mr Sunak, meanwhile, steered clear of the annual gathering in the exclusive Swiss ski resort.