Volodymyr Zelensky is due to meet Rishi Sunak in the UK on Monday in the latest stop on a tour of European capitals by the Ukrainian president.
The two leaders will conduct “substantive negotiations” amid the Russian invasion, Mr Zelensky said.
The visit will make the Ukrainian president the first world leader whom Mr Sunak has hosted at his grace-and-favour country residence.
It follows meetings in Paris, Berlin and Rome, and comes three months after his first trip to London since the start of the war.
That occasion saw the Ukrainian leader attend an audience with the King and address Parliament in February.
The Prime Minister said: “This is a crucial moment in Ukraine’s resistance to a terrible war of aggression they did not choose or provoke.
“They need the sustained support of the international community to defend against the barrage of unrelenting and indiscriminate attacks that have been their daily reality for over a year.
“We must not let them down. The frontlines of Putin’s war of aggression may be in Ukraine but the fault lines stretch all over the world. It is in all our interest to ensure Ukraine succeeds and Putin’s barbarism is not rewarded.
“That is why the UK is sustaining our support to Ukraine – from tanks to training, ammunition to armoured vehicles. And this message of solidarity will ring loud in all my meetings with fellow world leaders in the days ahead.”
Mr Zelensky tweeted: “Today – London. The UK is a leader when it comes to expanding our capabilities on the ground and in the air.
“This cooperation will continue today. I will meet my friend Rishi. We will conduct substantive negotiations face-to-face and in delegations.”
The latest visit comes days after Liverpool hosted Eurovision on behalf of Ukraine.
It also comes after Mr Sunak and Defence Secretary Ben Wallace announced that the Government had agreed to authorise the donation of long-range cruise missiles after continued Russian attacks.
That decision was warmly welcomed by Mr Zelensky as a “significant enhancement” of Ukraine’s efforts in the war.
The visit by Mr Zelensky comes ahead of a G7 gathering in Hiroshima, Japan, later this week which will be attended by Mr Sunak.