The EU could impose tariffs on UK fish and agricultural goods in just seven days if Boris Johnson goes ahead with moves to disapply parts of the Northern Ireland Brexit protocol, legal experts have said.

The short, sharp shock is one of the three key retaliatory weapons available through the trade agreement, according to Catherine Barnard, a professor of EU law at Cambridge University.

1. The nuclear option – end the trade and cooperation agreement (TCA) using articles 770 and 779

These clauses allow the EU to terminate the entire trade agreement, spelling the end of tariff-free trade in both directions along with all the other elements of the deal, including 90-day visa-free holidays, and the fishing agreement.

It would essentially return the UK to a no-deal Brexit scenario, with damaging consequences including the suspension of police and security cooperation, a serious move with long-term consequences for EU-UK relations.

As this requires a year’s notice, it may not appeal to member states who want to show they have real teeth in the face of what they consider an act of bad faith by the UK.

2. Finger-on-the-button option – article 521

This would allow the EU to suspend the trade parts of the TCA, leaving all the other areas agreed last December, including visa-free holidays and police cooperation, intact.

Again, this option may not appeal to member states as it would not deliver the practical objectives to demonstrate that the EU has teeth.

“It seems to me unlikely that they would do this because, frankly, if things have got so bad that the EU is talking about terminating part of the treaty it seems unlikely that they carry on cooperating in the other areas,” says Barnard.

3. Trade war in a week – article 506, paragraph 2

This allows the EU to “suspend, in whole or in part”, access to its waters.

Such a response may have nothing to do with Northern Ireland, but Barnard says: “The advantage from the EU’s point of view is that you only have to give seven days’ notice,” so a trade war could be started within a week.

Moreover, article 506 allows for wider retaliation if deemed necessary. If the EU considers a suspension of fishing around the Channel Islands or the Isle of Man “commensurate to the economic or societal impact of the alleged failure” of the UK to comply with the threat, it can suspend tariff-free trade “in whole or in part”.

In other words, it could slap tariffs on fish and other goods within seven days.

But is it an either/or situation?

The treaty governing current trading arrangements – the trade and cooperation agreement – gives considerable powers to either side to terminate the relationship. Barnard says: “They [the EU] could do them all at the same time but it is more likely that they may try to escalate matters.”

For the EU to be considering pressing the button on any of the options just 18 months after the UK left the bloc with a trade deal is remarkable in the history of trade disputes.

“If you look at the World Trade Organization, the number of disputes between states is relatively small,” Barnard says. “The whole purpose of dispute resolution mechanisms is to resolve arguments, and that’s why you have those provisions in the withdrawal agreement and the TCA. But instead of talking about resolution, we are talking about ratcheting up the arguments to the point of terminating the treaty. It is extraordinary.”