By Alistair Grant

DEEP within the glassy, bureaucratic edifices of central Brussels, through the security gates and along bright, functional corridors, weary officials are gearing up for another busy year.

“I think we’re not allowed to talk about Brexit anymore,” one joked. “It’s now ‘the future relationship with the United Kingdom’.”

We may be leaving the EU tonight, but the upheaval of the last few years is far from over. In many ways, the end is barely even in sight.

Months of negotiations lie ahead as Boris Johnson and European leaders thrash out a future relationship over issues such as trade and security. 

Behind the scenes, officials in Brussels are clear about the challenges looming on the horizon. 

And while the Prime Minister insists the transition period will end on December 31 this year, there is a suspicion several issues will run over into 2021.

The European Commission will put forward its draft mandate for the future negotiations in the coming days, which will then need to be approved by the 27 remaining member states. 

“We probably won’t start formal negotiations until the end of February, beginning of March,” said one senior source, setting out the tight timescale.

“Then we have until – I’m being very approximate here – September, October, because you need to leave time for whatever agreement we reach to be ratified. 

“And in between that time, you have the deadline of July 1 for a very important decision to be made – and that is the decision on whether or not we extend the transition period.”

They added: “Time is extremely, extremely short, but we can only do what we can during that time.

“It may require us prioritising certain things, and it may require certain elements to be pushed into the future as well. We will just have to wait and see.”

The official said there are existing contingency measures in some areas, or international fallback options.

“I think in terms of priority, we’re going to be looking at security and defence cooperation, the economic relationship, fisheries, all that sort of stuff,” they added.

“But it may be the case that certain things aren’t done by the end of the year, and they will just have to continue. 

“And if those negotiations continue in 2021, that’s possible but you’re going to be in a very different situation, because the UK will be outside the single market and customs union. 

“We’ll have to be negotiating, but in a much more stressful environment, because there will be no status quo, as it were – the UK will be outside.”

Security will be a big issue going forward. 

The UK played a key role in setting up a new super-database of seven different systems intended to protect borders and stop terrorists or organised crime gangs slipping through the net.

But after Brexit, it risks being denied access to this. 

Fears have also been raised over the loss of access to European Arrest Warrants (EAW) and Europol, the EU’s law enforcement agency.

One Brussels official described Europol as “a bit of a Scottish colony”, given the crucial role Scots have played in its operations. 

Another said: “The UK is a serious security player, therefore there must be an interest from the EU side to have a relationship. 

“But there hasn’t ever been anything like this, so there are no obvious third country precedents. 

“Even the precedent with the US, which is probably a closer security relationship than with any other third country is not quite analogous.”

Everyone agrees it’s in the interests of both sides to cooperate. But officials say it’s not that simple. 

“Just because something is logical, which it is, and even wanted, which I suspect it is, does not mean it’s easy, and it isn’t easy because this is a legal entity,” one said.

“The EU is a legal entity more than it’s a political club.

“And therefore there are a whole set of rights and responsibilities in that, and if you decide that you are leaving certain things – like the jurisdiction of the court – then that creates a lot of things that you have to then find a workaround [for].”

Fishing is another crucial area, and there has been speculation the UK will trade access to its waters in return for a deal on financial services.

Downing Street dismissed this when it was recently suggested by Irish Taoiseach Leo Varadkar.

But as one Brussels trade official put it, other EU countries land about €600 million-worth of fish in UK waters.

There are half a dozen or so EU member states, including France, with a powerful interest in maintaining access. It's a useful illustration of where an agreement could be struck.

And what of Scotland? Nicola Sturgeon is due to set out her next steps on independence today, following Boris Johnson’s rejection of her request for the power to hold a second referendum.

There is clearly goodwill in Brussels towards Scotland.

But officials are understandably loath to say anything concrete on the possibility of an independent Scotland rejoining the EU.

“You might want to ask somebody else that question,” replied one. “I am a mere functionary.” It’s a question that could become increasingly pressing.