Boris Johnson has flown to Kyiv for “surprise” talks with Ukrainian president Volodymyr Zelenskyy.
Downing Street said the prime minister made the unannounced trip “in a show of solidarity with the Ukrainian people”.
The lightning visit was revealed in a tweet by the Ukrainian embassy in London, which showed Johnson holding talks with Zelensky.
Above the picture was a winking emoji and “surprise”.
The trip had been shrouded in secrecy due to security concerns.
Afterwards, it was announced that the UK has agreed to provide Ukraine with 120 armoured vehicles and new anti-ship missile systems.
That is on top of the £100 million worth of high-grade military equipment announced yesterday, including more Starstreak anti-aircraft missiles, another 800 anti-tank missiles, and high-tech loitering munitions for precision strikes.
The UK is also guaranteeing £385m in World Bank lending to Ukraine, taking its total loan guarantee to up to $1 billion.
The PM said: “It is a privilege to be able to travel to Ukraine and meet President Zelenskyy in person in Kyiv today.
“Ukraine has defied the odds and pushed back Russian forces from the gates of Kyiv, achieving the greatest feat of arms of the 21st century.
“It is because of President Zelenskyy’s resolute leadership and the invincible heroism and courage of the Ukrainian people that Putin’s monstrous aims are being thwarted.
“I made clear today that the United Kingdom stands unwaveringly with them in this ongoing fight, and we are in it for the long run.
“We are stepping up our own military and economic support and convening a global alliance to bring this tragedy to an end, and ensure Ukraine survives and thrives as a free and sovereign nation.”
A Downing Street spokesperson said: “The prime minister has travelled to Ukraine to meet President Zelenskyy in person, in a show of solidarity with the Ukrainian people.
“They will discuss the UK’s long term support to Ukraine and the PM will set out a new package of financial and military aid.”
It is the first time the prime minister has visited Ukraine since the Russian invasion on February 24.
Number 10 later issued a picture appearing to show the two leaders on a walkabout in Kyiv.
The BBC reported that in a Facebook post, Andriy Sybiha, deputy head of the Ukrainian president’s office said: “The UK is the leader in defence support for Ukraine. The leader in the anti-war coalition. The leader in sanctions against the Russian aggressor.”