Boris Johnson has pledged to send 120 armored vehicles to Ukraine, after meeting President Volodymyr Zelensky for talks in Kyiv.
The Prime Minister said he made his unannounced visit to the Ukrainian capital as “a show of our unwavering support for the people of Ukraine”.
As well as the armored vehicles, in a significant stepping-up of UK military support, Mr Johnson said Britain would also be sending new anti-ship missile systems.
It comes the day after he announced a further £100 million worth of UK military assistance, including anti-tank and anti-aircraft weaponry and so-called “suicide drones”, which loiter over the battlefield before attacking their target.
Following his meeting with the Ukrainian leader, Mr Johnson said in a statement: “Ukraine has challenged 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 Zelensky’s resolute leadership and the invincible heroism and courage of the Ukrainian people that Putin’s monstrous aims are being thwarted.
“I made it 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 agreeing to a global alliance to bring this tragedy to an end, and ensure Ukraine survives and thrives as a free and sovereign nation.”
As well as the additional equipment Mr Johnson also confirmed further economic support, guaranteeing an additional 500 million dollars (£385 million) in World Bank lending to Ukraine, taking total UK loan guarantees to one billion dollars (£770 million).
The announcements follow an urgent appeal by the Ukrainians for more weapons – including armored vehicles – ahead of an expected Russian offensive on the eastern Donbas region, already part held by pro-Moscow separatists.
The inclusion of anti-ship missile systems came as British military intelligence said Russian naval forces were continuing to launch cruise missile strikes in support of ground operations in the region.
There were no immediate details of the armored vehicles, although Defense Secretary Ben Wallace indicated during a visit to Romania on Friday that they would be Mastiff six-wheel-drive patrol vehicles.
According to the latest intelligence assessment by the UK Ministry of Defence, Russian forces are continuing to hit non-military targets like the train station at Kramatorsk.
At least 52 people died and many more were injured when a missile struck the station which was packed with thousands of women and children seeking to flee westwards ahead of the Russian advance.
In his nightly video address, Mr Zelensky called on the international community to hold to account the Russian forces responsible.
“All world efforts will be directed to establish every minute of who did what, who gave what orders, where the missile came from, who transported it, who gave the command and how this strike was agreed,” he said.
The Kremlin has denied responsibility for the attack, claiming it was carried out by the Ukrainians themselves as a “provocation”.
However Western officials believe it was probably a Russian Tochka-U missile, which Nato refers to as a SS-21, which was fired indiscriminately towards the town center.
The Ukrainian authorities continued to urge civilians in the east to leave as Russian forces re-group following their withdrawal from the areas around Kyiv in the face of fierce Ukrainian resistance.
Deputy prime minister Iryna Vereshchuk said 10 evacuation corridors were planned for Saturday in the hope of allowing residents to leave war zones in the eastern regions of Donetsk and Luhansk, which comprise the Donbas, as well as neighboring Zaporizhzhia.
Meanwhile Downing Street declined to discuss details of Mr Johnson’s travel to Kyiv, citing security reasons.
His visit however coincided with the arrival of Austrian Chancellor Karl Nehammer, who also held talks with Mr Zelensky, while European Commission president Ursula von der Leyen was in the city on Friday.
Last month the prime ministers of Poland, the Czech Republic and Slovenia visited Kyiv in a show of support for the Ukrainians.