Spain's state prosecutor has called for charges to be brought against Catalan leaders.
The prosecutor has said charges of rebellion, sedition and misuse of public funds should be brought against the region.
The move comes as Madrid imposes direct rule on the region for the first time in 40 years.
Tensions have been growing in Spain after Catalonia held a controversial referendum vote, during which hundreds were injured as they went to the polls. The Catalan parliament later declared independence.
Prime Minister Theresa May said the UK "does not recognise" Catalonia's independence.
In a statement, she said: "The UK does not and will not recognise the unilateral declaration of independence made by the Catalan regional parliament.
"It is based on a vote that was declared illegal by the Spanish courts. We continue to want to see the rule of lasw upheld, the Spanish Constitution respected, and Spanish unity preserved.”