PA MediaSir Keir Starmer during talks with Ukrainian President Volodymyr Zelensky (left) in Armenia on SundayThe UK intends to begin negotiations to join a £78bn (€90) European Union loan scheme seen as providing vital support for Ukraine.
Sir Keir Starmer will tell the European Political Community (EPC) summit in Armenia on Monday about the UK's ambitions, which further signals the government's desire for closer ties with the EU.
The prime minister said the talks are aimed at strengthening Ukraine's defences while also trying to give UK firms access to future contracts.
He met Volodymyr Zelensky on Sunday in Armenia, with the Ukrainian president asking Sir Keir to pass on his "best regards and thanks" to King Charles III for his "strong words" in support of Ukraine during his recent state visit to the US.
The two leaders held talks in a hotel in Armenia's capital Yerevan ahead of the EPC summit, to which 48 heads of state and government have been invited.
Sir Keir said the UK is "ready to do the necessary work" with allies on Monday to give Ukraine the "support you need, the pressure you need to continue to put on Russia to make sure that we get a just and lasting peace, but one that is right for Ukraine".
Downing Street said further sanctions by the UK on Russian companies are expected to be announced this week, in a bid to disrupt military supply chains.
Ahead of the EPC meeting, which first convened in October 2022 months after Russia launched its full-scale invasion of Ukraine, Sir Keir said "we all reap the benefits" when the UK and EU work together.
He added: "In these volatile times we need to go further and faster on defence to keep people safe.
"That is why we are beginning negotiations with the EU to ensure Ukraine gets the equipment it needs to defend its freedom, while driving opportunities for British industry to play its full part."
EU leaders last month approved the deal to lend Ukraine €90bn over the next two years after Hungary lifted its veto, with the funding described as "a matter of life and death" by Ukrainian Deputy Prime Minister Taras Kachka.
Two-thirds of it will be spent on bolstering Ukraine's defence needs while the rest will go on broader financial assistance.
No 10 also said Sir Keir will discuss the situation in the Middle East with European partners at the EPC.
This is expected to include how they can contribute to security in the Strait of Hormuz.
Iran has severely limited traffic through the vital oil shipping channel in response to the bombing campaign by the US and Israel, which began in February.
Sir Keir has repeatedly outlined a desire to reset relations with Europe, including closer ties on security and the economy, but said it does not amount to a reversal of Brexit.
The Conservatives and Reform UK last month made clear their opposition to a government plan for legislation to allow the UK to adopt new EU laws without Parliament having to hold a full vote each time.

Sign up for our Politics Essential newsletter to keep up with the inner workings of Westminster and beyond.
War in UkraineKeir Starmer
