Volodymyr Zelensky was welcomed by Sir Keir Starmer with a warm hug and handshake outside No10.
The Ukrainian president arrived in Downing Street shortly before 9am.
Sir Keir said Mr Zelensky's visit was a "very special moment".
In a bilateral meeting with the Ukrainian President, the Prime Minister said the UK is "united in our Parliament" in supporting Kyiv for as long as possible.
"It's a very special moment for us to reiterate our support for you and the Ukrainian people in the fight against Russian aggression and to have this opportunity to confirm again our resolve to stand with you, so thank you so much for accepting the invitation," he said.
Mr Zelensky was due to become the first foreign leader to address Cabinet in person since 1997, an honour which underlines Britain’s steadfast support for Ukraine in its war against Vladimir Putin’s invasion.
The Ukrainian president will be the first official visitor to Downing Street under Sir Keir’s premiership, and the first foreign leader to address the Cabinet in person since US president Bill Clinton in 1997.
He was expected to brief ministers about the situation in Ukraine and the need to expand Europe’s defence industrial base, as well as agreeing a £3.5 billion defence export finance deal with Sir Keir.
The Prime Minister said: “Ukraine is, and always will be, at the heart of this Government’s agenda and so it is only fitting that President Zelensky will make a historic address to my Cabinet.
“Russia’s incremental gains on the battlefield are nothing compared with the collective international support for Ukraine, or the strength of ties between our people.”
Speaking to the BBC ahead of the meeting, Mr Zelensky said he needed “strong support” from Sir Keir and would push for permission to use British long-range missiles against targets in Russia, particularly locations from which Russian forces were attacking civilian targets in Ukraine.
During last week’s Nato summit, Sir Keir suggested Ukraine would be able to use the missiles against Russian targets, but Downing Street later backtracked – saying Government policy on the use of the long-range Storm Shadow missiles had not changed.
President Zelensky’s visit to Downing Street follows the European Political Community (EPC) summit at Blenheim Palace, Oxfordshire, on Thursday, at which 44 countries and the EU agreed to target Russia’s “shadow fleet” of tankers seeking to evade sanctions on Russian oil.
The group of around 600 vessels carries 1.7 million barrels of oil per day, providing funds for Moscow’s war effort while also allegedly acting as listening stations or transporting weapons for Russia.
The ships are often old and unsafe, and engage in dangerous practices such as turning off their location tracking systems, increasing the risk of a serious collision.
Sir Keir said: “Alongside our European partners, we have sent a clear message to those enabling Putin’s attempts to evade sanctions: we will not allow Russia’s shadow fleet, and the dirty money it generates, to flow freely through European waters and put our security at risk.”
Speaking at the EPC summit on Thursday, Mr Zelensky said the UK had been “one step ahead in its determination to support Ukraine” since the Russian invasion began.
He added: “This is the kind of resolve we need to stop Russian terror.”
On Thursday, the UK also sanctioned 11 oil tankers thought to be involved in evading sanctions, including the Rocky Runner, which previously sought to escape UK action by changing its operator.
Most of the ships sail under the flag of Gabon, although some – including the Rocky Runner – are flagged with the Cook Islands and one sails under the flag of Panama.
Sanctioned vessels are prohibited from entering UK ports and may be detained or directed to enter a port.
