MOST diplomats hoped that the Brexit vote in June 2016 would not impinge on security. After all, the issue had hardly featured in the campaign. All sides have a strong interest in continuing to work together. And European defence co-operation makes little sense without Britain, the biggest spender and one of Europe’s only two global military powers. Yet a row over Britain’s participation in the Galileo satellite positioning system could now upset all such calculations.

The British were for years hostile to Galileo, which they saw as unnecessarily (and expensively) duplicating America’s global positioning system (GPS). But once work on Galileo began, Britain became more enthusiastic, not least because its firms secured some of the juiciest contracts. The British are paying some 12% of the total cost of Galileo, which is expected to exceed €10bn ($12bn) by its completion in 2020. British-based firms have won back around 15% of this. Indeed, Galileo has become a driver of the thriving British space industry, which has an annual turnover of £14bn ($19bn)—half of it accounted for by satellite broadcasting companies—and entertains hopes of becoming the world’s biggest after America’s.

Galileo is run by the European Space Agency, which is not part of the EU and includes non-members such as Norway. But the European Commission provides the money and takes the key decisions on how it is spent. Its lawyers say these preclude contracts with providers outside the EU, so the commission wants to stop Galileo work by British-based firms after Britain leaves the union next March. It is also ensuring that non-members cannot block Galileo procurement decisions. After all, Brexit means Brexit, jokes one Eurocrat.

The commission also insists that, as a non-member, Britain cannot have full access to Galileo’s “public regulated service” (PRS), a militarily secure, unjammable part of the project. Norway and America have asked for access to the PRS. But the EU is resisting it, on the argument that letting non-members into such sensitive areas would undermine the club’s “strategic autonomy”, a new concept of questionable value. If Britain were allowed in, others would demand the same treatment.

This legalistic approach has proved a red rag to British ministers. Unless the EU changes its views on contracts and the PRS, the government threatens to pull out of Galileo altogether. Some continental companies might pinch contracts from British competitors. But losing their expertise would delay the project and could add as much as €1bn to the bill. Losing access to British ground-stations on Ascension Island and in the Falklands would be an annoyance. Britain’s suggestion that it might try to recoup the money it has invested in Galileo is unlikely to get far. But the threat that, if it is cut out of the programme, Britain might build a satellite positioning system of its own is more serious.

At least technically, it would be feasible, says Bleddyn Bowen, a space expert at Leicester University. He reckons it could be done at a cost of some £3bn-5bn. Some enthusiasts talk of sharing this cost with Australia or Japan, and getting a new system up and running almost as soon as Galileo itself. But Mr Bowen thinks the idea is essentially a bluff. The benefits from creating yet another satellite positioning system would be marginal, and the cost seems prohibitive when the defence budget is under immense pressure.

More worrying, says Sophia Besch of the Centre for European Reform, a think-tank, are the implications of going it alone for wider defence co-operation. In January 2017 Theresa May was criticised when she hinted that Britain’s future defence relations with Europe might be affected by the terms of any Brexit deal. The prime minister later backtracked by promising that Britain’s defence commitments were unconditional. But the threat to exclude Britain from Galileo has reopened the question. The government now says participation in Galileo is a strategic choice that “will have a permanent effect on our future defence and industrial co-operation.”

For in the end it is a question of trust. Britain is the closest military and security ally that many EU countries have. France and Germany want to involve Britain more deeply in building up Europe’s military capacity. If lawyers can obstruct future co-operation with the British over Galileo on security grounds, that sends a deeply unhelpful message for working together in other areas. That is why some EU countries think the commission is going too far in demanding British exclusion from the project. Joschka Fischer, a former German foreign minister, says Brussels is being “stupid” over Galileo.

The row could also worsen the legal and practical difficulties over broader co-operation on domestic security. The EU argues that post-Brexit Britain cannot remain a full member of Europol, the policing agency, or the European Arrest Warrant process for extradition. EU countries suspect that Britain may not always share their commitment to data privacy. Mrs May’s insistence on escaping from the jurisdiction of the European Court of Justice is another obstacle. Security was once thought to be one of the simplest parts of the Brexit negotiations. Now even it is proving to be trying.