Britain's departure from the EU is "an historic moment from which there can be no turning back", Theresa May has said. She was speaking after Britain's EU ambassador formally triggered the two year exit process by handing a letter to EU Council President Donald Tusk. A sombre-looking Mr Tusk said it was not a "happy day" for him or the EU. The European Parliament warned security cooperation was not on the table as it fired its opening salvo in what are expected to be tough negotiations. In her letter to Mr Tusk triggering Article 50 of the Lisbon Treaty, Mrs May said failure to reach a trade deal within the two-year time limit could "weaken" cooperation in the fight against crime and terrorism. Read the letter in full (pdf) But Guy Verhofstadt, the European Parliament's chief Brexit negotiator,…