Sir Keir Starmer will travel to Brussels on Wednesday to hold talks with EU leaders about "putting the Brexit years behind us" and bolstering ties with the UK.
The prime minister has vowed to "reset" relations with Europe following tensions between the previous Tory government and the EU - though he has ruled out a return to the single market, customs union or freedom of movement.
Downing Street said Sir Keir will be focused on delivering a "broad-based security pact" with the bloc, as well as tackling barriers to trade.
In discussions with the likes of European Commission President Ursula von der Leyen, he will say that at a time of growing instability in the world, it is important that "like-minded countries cooperate more closely on areas of shared interest".
Ahead of the meeting, the prime minister said: "The UK is undeniably stronger when it works in lockstep with its closest international partners. This has never been more important - with war, conflict and insecurity all knocking on Europe's door.
"We will only be able to tackle these challenges by putting our collective weight behind them, which is why I am so determined to put the Brexit years behind us and establish a more pragmatic and mature relationship with the European Union.
"Better cooperation with the EU will deliver the benefits the British people deserve - securing our borders, keeping us safe and boosting economic growth."
No concrete announcements are expected from the talks, which will also include meetings with European Council President Charles Michel and the European Parliament's President Roberta Metsola.
The European Commission said the discussions with Ms von der Leyen would be "the beginning of a conversation" about closer ties.
Sir Keir is under pressure to agree to Brussels' calls for a deal on youth mobility to allow young EU citizens greater flexibility to come to the UK to study and work and vice versa.
This is something the prime minister has so far resisted, saying red lines for the reset rule out a return of freedom of movement, alongside rejoining the customs union and single market.
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Before the talks, pro-EU campaigners pushed for a rethink, accusing the government of "letting young people down".
Sir Nick Harvey, chief executive of European Movement UK, said: "Dismissing the idea of reciprocal youth mobility simply means letting down British young people, who face all sorts of economic difficulties and have seen their horizons curtailed by Brexit.
"Young people want and deserve the chance to study or work in Europe. The government owes it to them to make sure they get that chance."
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Tap hereCal Roscow, from the Best for Britain campaign, said: "Brexit robbed young Brits of irreplaceable opportunities to experience new cultures, meet new people and learn new languages while working, travelling and studying in Europe.
"The new government has the chance to give these formative experiences back to young Brits, with this universally popular scheme that the European Commission is already open to agreeing."
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