This EU-UK summit has for months been openly billed by Sir Keir Starmer’s Downing Street as a hugely significant moment for this government.

The Labour leader promised in his 2024 election manifesto that the UK would sign a new security pact with the EU to strengthen cooperation and improve the UK’s trading relationship with the continent.

Since winning power in July, he has embarked on a charm offensive across European capitals in a bid to secure that better post-Brexit deal.

Monday is set to be when the PM makes good on those promises at a historic summit at Lancaster House in London.

Read more: What exactly could the UK-EU reset look like?

There, the EU and UK are expected to sign a security and defence partnership, which has taken on a new sense of urgency since the arrival of President Trump in the White House.

It is an agreement that will symbolise the post-Brexit reset, with the PM, European Commission president Ursula von der Leyen and European Council president Antonio Costa are also expected to sign off on a communique pledging deeper economic cooperation.

But, rather like the torturous Brexit negotiations I covered for years in London and Brussels under Conservative prime ministers, Sir Keir’s post-Brexit reset talks are going down to the wire.

As of 10.30pm on Sunday, discussions were set to continue overnight, the two sides snared up over details around fisheries, food trade and youth mobility.

It’s not that both sides don’t want the reset: the war in Ukraine and the spectre of the US becoming an unreliable partner have pushed London and Brussels closer together in their common defence interest.

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Fishing and youth mobility – the two snags

But the pressure for this deal weighs more heavily on our prime minister than his European colleagues. He’s been talking for months about securing a reset and better trading relationship with the EU to bolster the UK economy.

His need to demonstrate wins is why, suggests one continental source, the Europeans are letting talks go to the wire, with London and Brussels in a tangle over fishing rights – key demands of France and the Netherlands – and a youth mobility scheme, which is a particular focus for Berlin.

“The British came with 50 asks, we came with two – on fishing and the youth mobility scheme,” says one European source.

The EU is asking for longer-term access to UK fishing grounds – a 10-year deal – which the British government has rebuffed, insisting it will not go beyond a four-year deal.

In response, Brussels is saying it will not lift regulatory checks on food, agricultural and animal products unless the UK moves on fishing. This has left the two sides at an impasse.

EU sources say Brussels had offered a time-limited deal to lift checks on animal products – replicating London’s offer on fisheries – but the UK is reluctant to do this as it leaves too much uncertainty for farmers and supermarkets.

Scotland election weighing on talks

A deal on food products, known as sanitary and phytosanitary (SPS) goods, would be a boost for the economy, with potentially up to 80% of border checks disappearing, given the breadth of products – paint, fashion goods, leather as well as foods – with an animal component.

Any deal would also mean the UK would have to align with rules made in Brussels and make a financial contribution to the EU to fund work on food and animal standards.

Both elements will trigger accusations of Brexit “betrayal”, as the UK signs up as a “rule taker” and finds itself paying back into the EU for better access.

Government figures had been telling me how they were more than prepared to face down the criticisms likely to be thrown at them from the Conservatives.

But sensitivities around fishing, particularly in Scotland, where Labour is facing elections next year, have weighed on talks.

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The other area of huge tension is over a youth mobility scheme, which would enable young adults from member states to study and work in the UK and vice versa.

Government sources familiar with the talks acknowledge some sort of scheme will happen, but want details to be vague – I’m told it might be “an agreement about a future agreement”, while the EU sees this a one of its two core demands.

In talks late on Sunday night, the UK government appeared to be softening on re-opening the pre-Brexit Erasmus student exchange scheme as perhaps a way to get around the impasse, according to one EU source.

The UK rejoining this scheme had been rebuffed by Sir Keir last year, but was raised again last night in talks, according to a source.

Common ground on defence and security

Wherever the economic horsetrading lands, the two sides have found common ground in recent months is on defence and security, with the UK working in lockstep with European allies over Ukraine and relationships deepening in recent months as Sir Keir Starmer has worked with President Macron and others to try to smooth tensions between Kyiv and Washington and work on a European peace deal for Ukraine.

If details on trade, youth mobility and fisheries are fudged on Monday, the expectation is that the two sides will sign a security partnership that will reiterate the UK’s commitment to build up the continent’s defence capability and stand united against Russian aggression with its partners.

The deal should also mean British arms companies will be able to access the EU’s €150bn rearmament programme, which has been set up to create a massive surge in defence spending over the next five years as Europe prepares itself to better repel threats.

As I write this, talks are ongoing, but it is clearly in neither side’s interest for Monday to go wrong.

The EU and UK need to maintain a united front and, more importantly for Keir Starmer domestically, the PM needs to show an increasingly sceptical public he can deliver on his promises.

Easing trade barriers with Britain’s biggest trading partner and signing an EU defence pact would be two manifesto promises delivered.

And with his popularity sinking to a record low in recent days, he could really do with a win.