U.K. Prime Minister Theresa May got a first taste of the high-wire act involved in European Union negotiations after her opening offer for a Brexit deal ran into immediate opposition from EU leaders who challenged her attempt to tie a free-trade deal to security cooperation.

In an early sign that two years of divorce negotiations will be as contentious and complicated as feared, Guy Verhofstadt — the European Parliament's point person on Brexit matters and someone who will have a say on the final outcome — complained that “security is far too important to start to bargain it against an economic agreement.”

The slap came hours after May formally served notice that the U.K. will leave the EU in March 2019 and immediately pitched for an unprecedented trade deal that would enable banks to maintain easy access to the regional economy. Her government will begin the parliamentary process of taking back control over thousands of EU laws on Thursday.