LONDON (Reuters) - Prime Minister Keir Starmer will use his role as host of a European forum on Thursday to press his desire to reset Britain's post-Brexit relationship with the European Union and to reaffirm support for Ukraine.

Two weeks after winning a large majority in an election and fresh from his first international trip to NATO in Washington as prime minister, Starmer hosts the European Political Community (EPC) of more than 40 nations at Blenheim Palace, the birthplace of Winston Churchill, to build cooperation on border security.

The one-day meeting of a group set up after Russia's invasion of Ukraine in 2022 provides Britain's new leader another opportunity to push for closer defence cooperation especially before the possible election of Donald Trump who suggests that Europe must do more to protect its own security.

"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," Starmer said in a statement.

"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."

Starmer has been determined to reset EU ties after the 2016 vote to leave the bloc created years of rancour. He has said he does not see Britain rejoining the single market or customs union in his lifetime.

A decision by his predecessor Rishi Sunak to call an early election means Starmer can use NATO and the EPC to advance early talks.

Officials stress that the talks are very much in their infancy, and the ultimate goal of negotiating a security pact, covering a broad range of areas such as energy, supply chains, pandemics and migration will come much later in negotiations when the EU has been clear there will be no "cherry picking".

But the EPC offers Starmer, flanked by foreign minister David Lammy and Nick Thomas-Symonds, his minister for EU relations, another chance to hold meetings in numbers it would normally take a new government months to set up.

In Washington, defence minister John Healey said the Labour team there had "met and talked with every one of the 32 member (NATO) nations, Ukraine of course ... and the Indo Pacific four nations that are here as well".

MIGRATION AND MOLDOVA

At Blenheim Palace, the leaders, including Ukrainian President Volodymyr Zelenskiy, are expected to not only express renewed support for Kyiv but also to discuss energy security, safeguarding democracy and migration.

Some critics say the EPC is little more than a talking shop, often offering no firm agreements, but others say its informal nature encourages wider conversations, which one German government source said strengthens "geopolitical cohesion between the EU and third countries in the region".

A French official said London wanted the EPC to call for action against the Russian shadow fleet of tankers, which avoid sanctions, by closing the loopholes, while Moldova, which borders Ukraine, and France would work on cooperation to fight against foreign interference.

Starmer would take part in the migration working group and the Moldova Group to underline the country's sovereignty and democracy in the face of Russian interference. He will also champion a move to deploy staff to a new rapid returns unit to take control of national borders.

But perhaps most important to Starmer is the development of personal relationships with other leaders, something he says is essential to the way he works to get the best results.

He hosted Irish Prime Minister Simon Harris at his country residence Chequers on Wednesday and will entertain French President Emmanuel Macron over dinner on Thursday. He will also have a number of bilateral meetings, including with Polish Prime Minister Donald Tusk.

"This meeting of European leaders is an opportunity to push on and begin delivering on the people's priorities," Starmer said.

(Reporting by Elizabeth Piper and Andrew MacAskill, additional reporting by Andreas Rinke in Berlin, John Irish in Paris and Andrew Gray in Brussels; Editing by Sharon Singleton)

By Elizabeth Piper and Andrew MacAskill