Catalonia in north-east Spain will issue a challenge to the EU as regional leader Artur Mas prepares to ask voters to declare whether they want an independent state within the EU.

A yes vote in the referendum would not just create a constitutional crisis for Spain, which has no mechanism for allowing the independence of one of its regions, but would also issue a clear challenge to the EU.

The EU has no system for the break up of a member state, and a new entity could have future membership blocked by just one member country.