Poland’s prime minister has said he hopes for the “Breturn” of the UK as a member of the European Union – as he discussed a defence treaty with Sir Keir Starmer.

Donald Tusk, who was hosting the prime minister for discussions on a UK-Polish defence pact in Warsaw, said it was his “dream” that “instead of a Brexit, we will have a Breturn”.

Standing alongside Sir Keir at a joint news conference, the Polish premier also said he had discussed greater cooperation between the UK and the EU.

Mr Tusk, who was the president of the European Council during the years that Britain left the EU, said: “For obvious reasons, we also discussed another issue, the cooperation between Great Britain and the European Union.

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“I’m sure you will recall when we learned about the results of the Brexit referendum. I was head of the European Council… at that time. My first emotional reaction was to say: ‘I already miss you.’

“I remember our press briefings as if it was yesterday. I already miss you, that’s what I said.”

He added: “This is not just about emotions and sentiments – I am aware this is a dream of mine, that instead of a Brexit we will have a Breturn.

“Perhaps I’m labouring under an illusion. I’d rather be an optimist and harbour these dreams in my heart – sometimes they come true in politics.”

Sir Keir, who held the role of shadow Brexit secretary while Labour was in opposition and backed remain in the 2016 referendum, has repeatedly ruled out rejoining the customs union or single market.

He has, however, said he wants to deepen post-Brexit ties with Brussels

Mr Tusk was speaking after Sir Keir travelled to Poland to discuss a defence pact with the NATO ally – which Mr Tusk said he hoped would be ratified “this year”.

The new treaty is designed to protect Europe from Russian aggression, tackle people-smuggling gangs, and combat misinformation and cyber threats.

Sir Keir was also asked whether the UK’s attendance at a defence summit in Poland earlier this week meant he was in favour of “creating an army” for Europe – to which he replied he was not.

Asked about the E5 defence ministers meeting in Warsaw and whether he supported creating a common European army, Sir Keir said: “The meeting that happened the other day is vitally important. That isn’t about creating armies.

“It’s about how we share our security concerns and build on what we’ve already got.”

As part of the defence pact, a £4bn partnership for new air defence systems in Poland has been agreed. The project will be headquartered in Bristol.

“The UK has secured £8bn of defence deals in Poland over the last three years alone, and we’re going further today, opening a new joint programme office in Bristol to deliver our £4bn partnership, to deliver the next generation of air defence systems to Poland,” he said.

He added: “We share an unbreakable commitment to NATO and an unbreakable commitment to Ukraine.”

During his visit to Poland, Sir Keir also made his first visit to Auschwitz, which he described as “utterly harrowing”.

The prime minister visited the former Nazi concentration camp, where he laid a wreath ahead of the 80th anniversary of its liberation.

After he and his wife Victoria, who is Jewish, visited the site, Sir Keir said: “Nothing could prepare me for the sheer horror of what I have seen in this place. It is utterly harrowing.

“The mounds of hair, the shoes, the suitcases, the names and details, everything that was so meticulously kept, except for human life.”

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His visit to Poland came following a surprise trip to Kyiv on Thursday, where he reiterated his support for Ukraine and suggested that British troops could be deployed to the country as part of peacekeeping efforts.

In an exclusive interview with Sky News’ political editor Beth Rigby, Sir Keir said the UK would play its “full part” in any peace negotiations – including by deploying British troops for peacekeeping – though added that he did not want “to get ahead of ourselves”.

During his visit, Sir Keir also met Polish businesses, including the firm InPost which has announced it will invest a further £600m into the UK in the next five years to grow its operations.

It is thought that the overall £1bn investment by the firm, which operates parcel lockers, could support up to 12,000 new jobs.