Sir Keir Starmer joined a meeting on Ukraine with the United States and European leaders at a major defence summit.
Efforts to end the war between Ukraine and Russia are high on the agenda at the Munich Security Conference as the four-year anniversary of Vladimir Putin’s full-scale invasion nears.
The future of transatlantic ties is also the focus of discussions as Europe grapples with antagonism from Donald Trump’s White House.
The US delegation is being led by US Secretary of State Marco Rubio, who pointed to a “new era in geopolitics” before his arrival.
The Prime Minister was seated next to Mr Rubio at the Ukraine meeting, although the US politician had not arrived by the time the press left the room for the private talks to commence.
German Chancellor Friedrich Merz opened the session with a special welcome for Ukrainian President Volodymyr Zelensky.
Leaders from France, Denmark, Nato, the European Council, European Commission and other countries were also present.
The meeting was billed as an extension of talks held in Berlin in December, when European leaders set out conditions for any peace deal with Russia, including security guarantees backed by the US.
Another round of US-brokered negotiations between Ukraine and Russia to end the war are set to take place next week.
Sir Keir is also due to meet Mr Merz and French President Emmanuel Macron on Friday afternoon, before addressing the summit on Saturday morning.
The conference is being held at a time when the US’s commitment to Nato has been called into question.
Tensions flared over Mr Trump’s recent threat to take over Greenland from Nato partner Denmark.
Mr Merz called for a “new trans-Atlantic partnership” and acknowledged that “a divide” had emerged between the continents as he opened the conference.
US vice-president JD Vance’s criticism of European countries, including the UK, over free speech and immigration last year set the tone for a dramatic deterioration in relations.
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