Sir Keir Starmer will attend a meeting with US President Joe Biden and the French and German leaders in Berlin this weekend.
In the meeting of the "Quad" nations, the leaders will discuss the "concerning developments" in the Middle East and the situation in Ukraine, Sir Keir's spokesman said.
The meeting has been organised to coincide with Mr Biden's state visit to Germany.
It has been billed as a meeting of the Ramstein group of more than 40 nations that supply Kyiv with arms, named after they met at the US' Ramstein air base in Germany in 2022 to discuss Ukrainian defence capabilities after Russia invaded.
The gathering on Saturday will be the first time the group has met at a leader level.
The summit is meant to emphasise to Russian President Vladimir Putin that he cannot hope to play for time and wait for Western support for Ukraine to cease, a German government source said.
French President Emmanuel Macron and German Chancellor Olaf Scholz will meet the US and UK leaders in Berlin before they travel to the Ramstein air base in west Germany.
Sir Keir's spokesman said the situation in the Middle East will also be discussed as the conflict between Israel, Lebanon, Iran and Gaza continues to escalate.
Israel expanded its ground military operation into Lebanon last week, while the Lebanese militant group Hezbollah has launched more rockets against its southern neighbour.
Sir Keir has visited the US three times since becoming prime minister in July, speaking to President Biden about the West's approach to Ukraine and the Middle East.
Ukrainian President Volodymyr Zelenskyy has said the water entered a "very important phase" after his military struck an important Russian oil terminal.
Mr Putin's forces continue to occupy large areas of Ukrainian territory more than two and half years after Russia invaded.
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