ANOTHER week, another cabinet row over a hard or soft Brexit and how much cash to send to Brussels. Theresa May will set out new parameters for Britain’s departure from the European Union in a speech in Florence on September 22nd. In the meantime, one thing all sides accept is the importance of Britain and the EU co-operating on crime, security and counter-terrorism: a necessity reinforced by the latest, botched terrorist bombing of a London Tube train on September 15th. Yet post-Brexit this may be hard.

The government has just published its own plan, for a new treaty with the EU that essentially retains the current system of co-operation. This would perpetuate British participation in various security arrangements: Europol for police work, the European Arrest Warrant for extradition, the Eurojust system of judicial co-operation and continued access to information about those granted visas for the Schengen frontier-free travel area, as well as other security databases. Home Office officials concede that negotiating such a treaty is “ambitious”, but see no reason why it cannot be done.

That is less evident to the rest of Europe. All these bodies are EU agencies, governed by EU law, and ultimately fall under the jurisdiction of the European Court of Justice (ECJ), which Britain insists it will escape after Brexit. True, third countries such as America and Australia have observer status in Europol. Norway and Iceland are part of the European Arrest Warrant. And Denmark, which like Britain has an opt-out from EU justice and home-affairs policies, has secured access to Europol and the Schengen and other security databases. But Denmark is an EU member and, like Norway and Iceland, is in the Schengen area. All three also pay into the EU.

Moreover, the government wants a closer relationship than any of these models. Its paper says adopting them would produce a “limited patchwork of co-operation falling well short of current capabilities”. Hence its demand for a more comprehensive, bespoke treaty. But this is made harder by its red lines of ending any role for the ECJ and not paying large sums to the EU. The court will continue to adjudicate on EU laws, including those affecting security agencies. It also governs data privacy.

It might be possible to set up a joint arbitration system and even allow EU referees to monitor British data protection, but negotiating this would take time. Treaties always do. Camino Mortera-Martinez of the Centre for European Reform, a think-tank, notes that Norway and Iceland took five years to negotiate a treaty on the European Arrest Warrant, and it has taken a further eight years to be fully ratified.

One response in London to such obstacles is to stress the benefits to all sides from co-operation. Britain is a heavy user of the European Arrest Warrant and an important supplier of intelligence and other data to the EU, just as it spends the most on defence. Indeed, in Brexit talks ministers have at times played up this “security surplus” as if it were a bargaining card to be used to extract trade concessions. Last March Mrs May warned the EU that failure to agree a Brexit deal might weaken collaboration on security. The government’s latest paper eschews crude threats, but it is replete with examples of the EU benefiting from British expertise and knowledge.

The second answer, as with almost everything Brexit-related these days, is to propose a transition during which current arrangements continue. The Home Office is keen to avoid gaps in which criminals or terrorists might be able to evade arrest or extradition. Even that may not be simple, however. Steve Peers, a law professor at Essex University, points out that some countries have constitutions barring extradition of their nationals save to other EU members. They will not change their laws merely to satisfy post-Brexit Britain.

As a former home secretary, Mrs May is acutely aware of the value of security co-operation in Europe. In April 2016 she declared that remaining in the EU would make Britain “more secure from crime and terrorism”. To underpin that safety after Brexit, she will surely have to soften her pledge to escape the ECJ. And, as her government seeks yet again to replicate its current deals with the EU, some may wonder why it persists with Brexit at all.