By voting to leave the EU, the UK triggered the world’s most complicated divorce. But that’s not the only break-up on the cards.

Brexit has highlighted the deep divisions across the United Kingdom. While England and Wales voted overwhelmingly to leave the EU, everywhere else wanted to stay in. That includes Scotland and Northern Ireland, but also overseas territories like Gibraltar.

Unsurprisingly, Scottish first minister Nicola Sturgeon confirmed that “the option of a second independence referendum is on the table” following the Brexit result. (The first was held in 2014, and failed.) And Sinn Fein, the Irish republican party that campaigns for the reunification of Ireland, has seized on this political opportunity to call for a referendum on a united Ireland.

While the political dust settles, leaders in Scotland and Gibraltar are also looking at another, more middle-ground option: a federated membership of the UK, which they hope will allow them to retain their EU status whilst still remaining within the United Kingdom.

Senior figures in the Scottish Labour party are reportedly consulting with constitutional lawyers to look into a new federal system. And it’s not just devolved regions interested, the Labour initiative is also exploring whether the federal system could also be applied to English regions that voting to stay in the EU, such as London. The mayor, Sadiq Khan, has acknowledged that the British capital needs more say.

“As much as I might like the idea of a London city state, I’m not seriously talking about independence today,” Khan said. “I am not planning to install border points on the M25. But on behalf of all Londoners, I am demanding more autonomy for the capital—right now.”

There is perhaps some precedent for this proposed federal system. The Danish Realm is made up of three countries: Denmark, Greenland, and the Faroe Islands. Denmark joined the European Economic Community, the predecessor to the EU, in 1973. By 1979, Greenland gained autonomy from Denmark and seceded from the EU in 1985. The Faroe Islands have also chosen to remain outside the EU.

While there’s plenty of excitement from the political left for this federal structure, it’s still unclear how easily this so-called “reverse Greenland” can be applied in the UK.