"Then we meet and we start", U.K. Brexit Secretary David Davis said last week. Mr Tusk said recently that European Union leaders would respond within "more or less 48 hours" of the letter, starting possibly the most complicated series of negotiations in British history.
Conservative Mr Stevenson said it was time to start the process.
May is seeking to emphasise Britain's contribution to European security through training, cyber security and martime patrols as she begins the Brexit divorce process. Also of concern will be issues such as the status of E.U. property that may be located in the United Kingdom and vice versa, the status of various trade agreements that make up the European Union, and the status of citizens of the E.U. who have moved to the United Kingdom for work or other reasons and the status of British citizens who have moved to E.U. countries for work other reasons.
The ambitious two-year timetable has been questioned by the former directory of the World Trade Organisation.
Sources said European Union leaders would not be ready to tackle the issue at a previously planned summit on 6 April, as it would take four to six weeks to get ready after they received the UK's notification.
Mrs. May has prioritized the desire to control immigration and to reject the authority of the European Court of Justice, effectively ruling out membership in the European Union's customs union and its single market in goods and many services.
The official notification will declare that negotiations should begin as soon as possible, May's spokesman said. The Prime Minister informed EU Council President Donald Tusk on Monday morning that she will invoke the Article, ending months of speculation on the matter.
"Theresa May has repeatedly said that she wants to build a national consensus on Brexit, but it is increasingly clear she has failed to do so".
One of key issues is the €50 billion bill the United Kingdom must settle to leave the EU.
The United Kingdom held a referendum last June in which Britons voted by a 52-48 percent margin to leave the European Union, the first member state ever to do so.
Hannah White, IfG director of research, said, "The legislation required for Brexit will leave little parliamentary time for anything else - and making a success of it will require a large volume of bills and secondary legislation to be passed by Parliament against a hard deadline". Britain believes it can negotiate the exit agreement and a deal on future relations within the two-year negotiating period, although diplomats are sceptical.
Negotiations will also soon hit a fundamental topic: Britain wants "frictionless" free trade, but says it will restore controls over immigration, ending the right of European Union citizens to live and work in Britain. As EU Referendum's Richard North observes, those arrangements are "almost certainly going to require some oversight by the ECJ [the EU's court], while putting [Britain] in the position of having to accept everything thrown at [it] for those ten years, with the proverbial "no say" in the formulation of new rules".
The call would take advantage of polls showing May's Tories well ahead of the opposition Labour Party, which has been beset by internal strife under left-wing leader Jeremy Corbyn.
"The right to remain for these non-UK EU citizens must be guaranteed at the earliest opportunity to provide reassurance to smaller firms and their work forces".