The prime minister will welcome at least 45 European leaders to Blenheim Palace in Oxfordshire for a meeting of the European Political Community (EPC), which was set up in the wake of Russia's invasion of Ukraine two years ago.

All European countries - barring Russia and Belarus - are invited to the summit, intended to galvanise support for Ukraine, where fighting is expected to intensify over the summer.

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It comes just days after the attempt on Donald Trump's life in the US appears to have transformed the race for the White House, and raised concerns about a Republican administration cutting financial support for Kyiv.

But the summit is also seen as a golden opportunity for the new government to discuss a European security pact, and easing burdens on trade with the EU - two of Sir Keir's stated priorities which Downing Street said might otherwise have taken months.

Ahead of the summit, the prime minister said the Europe-wide challenge of illegal migration and people smuggling gangs was one the UK would take a leading role in addressing - an issue Rishi Sunak had put high on the agenda at the summit's previous meetings.

After scrapping the Conservatives's plan to send failed asylum seekers to Rwanda, Sir Keir will announce the redeployment of 100 Home Office staff working on that policy to a new "rapid returns unit" to send those ineligible to be in the UK to their home countries.

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Sir Keir will say: "We cannot let the challenges of the recent past define our relationships of the future. That is why European security will be at the forefront of this government's foreign and defence priorities, and why I am focused on seizing this moment to renew our relationship with Europe.

"The EPC will fire the starting gun on this government's new approach to Europe, one that will not just benefit us now, but for generations to come, from dismantling the people smuggling webs trafficking people across Europe, to standing up to Putin's barbaric actions in Ukraine and destabilising activity across Europe.

"My government was elected with a mandate for change. I asked the British people to judge me by my actions, not words. This meeting of European leaders is an opportunity to push on and begin delivering on the people's priorities. We will only be able to secure our borders, drive economic growth and defend our democracies if we work together."

Last week, announcing his priorities for the summit, Sir Keir said Europe cannot be "spectators in this chapter of history".

Blenheim Palace, a UNESCO World Heritage Site, is symbolic as the birthplace of Sir Winston Churchill who, ministers will point out, stood up to tyranny in Europe. It was also used by MI5 during the Second World War.

This is the EPC's fourth summit and arguably its most ambitious, with the leaders of NATO and the Organisation for Security and Cooperation in Europe also attending.

King Charles and Queen Camilla will host a reception for European leaders in the afternoon.

Sir Keir spoke about his interest in securing a defence and security pact with the EU in Washington this month, although he stressed that NATO was "still the cornerstone of our defence in Europe".

He has promised closer trade ties with the EU. In a sign of its importance, he has also created the post of minister for European relations, and given it to his ally Nick Thomas-Symonds.

Although no formal statement is expected at the end of the summit, several one-on-one meetings are planned.

On Wednesday night, Sir Keir met the Irish Taoiseach Simon Harris, and he will have dinner with Emmanuel Macron at Blenheim. He is also expected to meet with the prime minister of Poland.

Sir Keir's focus on migration, just after announcing a Border Security Bill in the King's Speech to tackle smuggling gangs, reflects the issue soaring up Europe's agenda.

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The new Home Secretary Yvette Cooper said: "Criminal smuggling gangs are making millions out of small boat crossings and the Tories left us with gimmick rather than grip.

"We will work right across Europe to tackle this problem at source, going after those profiting from this awful trade and bringing them to justice."