LONDON (Reuters) - A Spanish navy patrol ship entered Gibraltar’s territorial waters without permission on Tuesday, the government of the British outpost on the southern tip of Spain said.

The future of Gibraltar has become the first big dispute of Brexit since Prime Minister Theresa May filed formal divorce papers last week, with the EU draft position on negotiations saying the application of any EU-UK trade deal to Gibraltar had be agreed between Spain and Britain.

“Illegal incursion into British Gibraltar Territorial Waters by Spanish Navy patrol ship Infanta Cristina this afternoon,” the government of Gibraltar said on Twitter, alongside a video of the ship.

On Monday, May’s spokesman played down comments by a former leader of her Conservative party, Michael Howard, that she would be prepared to go to war to defend the territory.