"It is based on a vote that was declared illegal by the Spanish courts. We continue to want to see the rule of law upheld, the Spanish Constitution respected, and Spanish unity preserved," the spokeswoman said in a statement.
Catalan lawmakers sharply escalated the political crisis in Spain by voting to declare independence, with Madrid immediately vowing to "restore legality" and quash the region's secessionist bid.
May's reaction mirrored those of other European leaders, including French President Emmanuel Macron and EU President Donald Tusk, who both voiced support for Spain's Prime Minister Mariano Rajoy.
The European Union and its member states have stood firmly behind Madrid in the standoff, triggered by Catalonia's controversial October 1 referendum on separation from Spain, deemed unlawful and void by Madrid.
However, a minister in Scotland's independence-minded devolved government said it respected the Catalan government action.