Boris Johnson is trying to reach a Brexit deal with the European Union, but his first effort at compromise looks too tough for Brussels to stomach.

According to two people familiar with the matter, the prime minister hasn’t ruled out keeping the contentious backstop arrangement for the Irish border, with one crucial condition: that the EU puts a clear time limit on it.

That idea is not something that the EU is formally considering, but it has been discussed as a potential back-up option, including by the bloc’s power axis of France and Germany, EU officials said earlier this week. Persuading the Irish to accept it would still be a considerable stretch.

One person familiar with Johnson’s thinking said the option of keeping the backstop but adding a time limit was the government’s Plan B. Another confirmed the prime minister isn’t ruling it out.

The so-called backstop is a set of arrangements designed to ensure that there are never any checks on goods crossing the border between the Republic of Ireland and Northern Ireland. It’s supposed to remain in place until a more permanent system can be introduced in the years after Brexit.