LKS 20170329 700026409; Britain's Prime Minister Theresa May, signs the official letter to European Council President Donald Tusk, invoking Article 50 and signalling the United Kingdom's intention to leave the EU, in the cabinet office inside 10 Downing Street on March 28, 2017.. British Prime Minister Theresa May will send a letter to EU President Donald Tusk with Britain's formal departure notification on Wednesday, opening up a two-year negotiating window before Britain actually leaves the bloc in 2019. LEHTIKUVA / AFP PHOTO / POOL / CHRISTOPHER FURLONG -- RESTRICTIONS / EMBARGO APPLY