What is the battle on the Northern Ireland Protocol all about?

Sources I speak to on the EU side suggest that any serious retaliation would only actually happen if the UK brings in a law allowing ministers to override post-Brexit border arrangements.

There will be lots of threats in meantime, and possibly restarting legal action, but any trade war would be a long way down line.

Also while there is a lot of heated talk right now of “ripping up the protocol” the UK is really talking about rewriting parts of it - with proposals it claims would respect integrity of single market (clearly there is big disagreement here).

The crux of the problem is that the UK is demanding Maroš Šefčovič, the EU's chief negotiator, seeks a new negotiating mandate - and sources in several EU countries make clear that is not something they could consider.

But some experts think there is more flexibility within current the mandate to explore ideas.

What that means is a fair bit of time. Samuel Lowe, of Flint Global, which advises business on policy, always has the best takes on these things and here is his beautifully sketched timetable.

What the UK seems to want is a system in which commercial data allows you to set up a “green” channel for goods that are clearly heading from Great Britain to Northern Ireland and are not a risk to enter the EU. And red channel for the rest.

But to EU that sounds like no border checks so they feel it's unacceptable.

Obviously neither side want a trade war (even with Jacob Rees-Mogg saying we wouldn’t retaliate very painful).

So one question is: is there anything that can move us further to that within current mandate.

The UK obviously doesn’t think so but some say there's already been fair bit of flexibility here.

On the argument that the UK signed this protocol and it was clear it would mean checks - clearly Boris Johnson did, one Whitehall source argued it was - in a way - a “moot point” because the reality now is that with unionists’ fury it is piling pressure on NI stability.

A Whitehall source told me: “The EU are failing to grasp this is about the Belfast Good Friday Agreement and peace and stability. If they understood that, they’d be more flexible.”

Want a quick and expert briefing on the biggest news stories? Listen to our latest podcasts to find out What You Need To know... 

To which an EU source close to the negotiations said the Protocol was all about protecting peace.

The EU has a long-standing and well-known commitment to peace and stability in Northern Ireland.

They said: "The very purpose of the Protocol, which was negotiated with and signed by the UK is to protect the Good Friday Agreement which is the cornerstone of that peace.

"Only joint solutions can work in this context and provide much-needed legal certainty to the people of Northern Ireland."

They also said that Northern Ireland stability was one of just three major priorities in Brexit negotiations despite the issue barely being focused on by campaigns during referendum.