BRITAIN is running out of time to finish negotiations before it is due to leave the European Union on March 29th 2019. Theresa May insisted in Parliament this week that the recent EU summit had made important progress. Yet the start of talks on a framework for future trade may not happen until after December. Hence a favourite idea for averting a “cliff-edge” Brexit: transition. This week Britain’s five biggest business lobbies demanded the government seek early agreement on a transitional deal to form a bridge between Brexit and a new trading arrangement.

Mrs May has conceded this in principle, although she still insists on calling it an “implementation” period. This is not just semantic. She upset businesses this week by suggesting that, unless a trade deal is done by next autumn, there may be nothing to implement. In practice, the need for transition is clear to all, which is why it is covered in the EU’s own negotiating guidelines. Yet the work on it that has now started will quickly hit problems that may cause delays. And timing matters. Philip Hammond, the chancellor, has noted that the value to business of a transition plan will diminish the longer it takes to agree.

A first issue concerns what rules to follow during transition. Mrs May has promised that businesses will not have to adjust twice to Brexit, which implies keeping current arrangements for now. The EU will insist that any transition prolong the acquis (ie, all existing laws) and the jurisdiction of the European Court of Justice (ECJ), but without British participation in EU institutions. Jean-Claude Piris, a former EU legal adviser, calls this a “full monty” transition, and suggests it will be the only version on offer. Yet it leaves questions. Will Britain be subject to future laws that it has no say in making? Could it temporarily retain a judge on the ECJ? What about annual EU budget decisions (including on Britain’s budget rebate) that are taken without any British vote? Or the controversial annual carve-up of fishing quotas?

More important may be legal issues. Some lawyers, including Mr Piris, reckon that transition can be agreed under Article 50 of the EU treaty, which governs the Brexit divorce. Yet the EU guidelines refer only to what is legally possible. Must transition be strictly time-limited to conform with withdrawal under Article 50—and what happens if it becomes necessary to roll it forward? If there is no agreement on even the framework for a future trade deal, can transition be legally workable at all, on the basis that under the article it needs to be a bridge to something?

There are also concerns about the EU’s treaties with third countries, including free-trade deals. Simply rolling these over when Britain is no longer formally a member may not be easy. Already third countries have objected to plans to divide up import quotas between Britain and the EU. And legal doubts exist also on the British side, since the withdrawal bill now going through Parliament will repeal the 1972 European Communities Act. Catherine Barnard, a Cambridge law professor who is a member of the UK in a Changing Europe academic network, points to uncertainty over the supremacy of EU law, created under section two of that act.

Might there be another route to transition? One idea is temporary membership of the European Economic Area (EEA), which includes Norway and Iceland. But this may not be easy to negotiate, and it would strain Mrs May’s promise to avoid two adjustments. Paul Daly, another Cambridge legal academic, believes the only legally watertight transition is to extend Article 50’s two-year deadline for Brexit. This can be done by unanimous agreement. Yet extension of EU membership beyond March 2019, which would preclude trade talks with third countries, might not be politically sellable in Britain.

For Brexiteers have a legitimate concern that transition could turn into a long-term prison which in effect keeps Britain in the club as a rule-taker, not a rule-maker. The EEA was originally conceived as a transitional arrangement, after all. And many suspect that Brexit will be betrayed. This week it emerged that a Tory MP (and party whip) had written to universities asking for details of how they taught students about Brexit. The search for enemies of the people who might thwart the voters’ choice in June 2016 continues.