Prime Minister Sir Keir Starmer is meeting Ukrainian President Volodymyr Zelensky in Downing Street.
The talks come as Zelensky seeks to secure renewed international support for his country's war effort as the conflict with Russia heads into its third winter.
Starmer is also due to meet Mark Rutte, the new head of the Nato military alliance, at No 10 later in the morning.
The UK has contributed £12.8bn to Ukraine in military and other aid since Russia launched its full-scale invasion in February 2022.
More than 200,000 Ukrainian refugees have temporarily settled in the UK.
Since taking office, the prime minister has continued the policy of the previous Conservative government in offering strong financial and diplomatic support for Ukraine's war effort.
It is the second time Zelensky has visited Downing Street since Starmer was elected prime minister. In July, the Ukrainian president became the first foreign leader to be hosted at No 10 since the change of government.
Starmer also met Zelensky in Ukraine in February, while he was still leader of the opposition, and held talks with him at a Nato summit in July.
That meeting of Nato countries saw the bloc promise Ukraine an "irreversible path" to membership, something the Ukrainian government considers vital for its security.
The Downing Street talks are part of a diplomatic tour for Zelensky, and he will also visit France and Italy later today.