"The Prime Minister reiterated that the UK is clear that the referendum had no legal basis and that any unilateral declaration of independence would be inconsistent with the rule of law," a spokesman for May said following her phone call with Spanish Prime Minister Mariano Rajoy.
"She added that the UK would not recognise any such declaration of independence by Catalonia," the Downing Street spokesman added.
Britain's message comes more than two weeks after Catalonia held an independence referendum on October 1, which was ruled illegal by Spain's Constitutional Court.
Since then there has been a stalemate between separatist leaders and the Spanish government, which has shaken stock markets and brought thousands of people onto the streets.
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London had previously said little on the Catalonia issue, with the foreign ministry on October 1 saying the referendum "is a matter for the Spanish government and people" while calling for the rule of law and constitution to be respected.
In Scotland, where voters rejected independence in a 2014 referendum, First Minister Nicola Sturgeon said she was "increasingly concerned" by the scenes of violence during the Catalonia vote.
"But a way forward that also respects democracy and the right of the people of Catalonia to determine their own future," she said last week.
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