The UK and the European Union have less than four weeks to agree to a trade deal following Brexit, but talks have stalled over “three critical issues”.

On Sunday, negotiators on behalf of British Prime Minister Boris Johnson and European Commission President Ursula von der Leyen resumed trade discussions in a final push to reach an agreement.

The two leaders will speak again on Monday.

But no matter how much talking happens between now and then, there is no guarantee the groups will strike a deal before the deadline.

So what are the key hurdles in their way?

And what happens if they miss the deadline?

According to the UK, any new agreement on fisheries must be based on the understanding that “British fishing grounds are first and foremost for British boats”.

This stance has become a major sticking point.

For some, reclaiming Britain’s bountiful fishing grounds is a symbol of sovereignty and an important benefit of leaving the EU.

But the EU, under pressure from its fishing communities, says a ‘UK First’ approach is unfair.

It’s worried about losing too much access to British waters and a potential crackdown on the types of fish they can catch and sell.

The issue is so important that the EU says a “fair deal” on fisheries is a precondition for a future free trade agreement on goods crossing borders.

Competition rules

Rules on state subsidies for business is another big issue.

The EU is worried the UK will give financial help to its businesses, which it argues would give them an unfair advantage.

In the eyes of Belgian MEP Philippe Lamberts, this problem was bigger than fishing rights.

“These are much more important and this is a very tough nut to crack, and it will really depend on whether Boris Johnson wants to limit the economic damage caused by Brexit,” Mr Lamberts said.

Policing

There is still debate about how the new rules of any agreement will be enforced.

Whose responsibility is it to catch rule breakers?

Who funds the governance of these rules?

How will they solve future disputes?

There is plenty to iron out.

What happens if they don’t reach a deal?

When the UK left the EU on January 31, it was given 11 months to negotiate a deal for life beyond Brexit.

Its time is nearly up.

If there is no deal by the deadline of December 31, border checks and taxes will be introduced for goods travelling between the UK and the EU.