On Wednesday, the Prime Minister set out her priorities for Brexit talks in a letter which triggered Article 50.
The EU's overall objective in the negotiations will be to preserve the Union's interests, those of its member states, its citizens and its businesses.
The House of Lords said in a report published earlier this month that the job of adapting the European Union regulations into British law is complicated by the "scale and complexity of the task", as well the fact that some decisions will be dependent on the result of Britain's negotiations with the EU. The remaining 27 countries want the negotiations to focus first and foremost on how the U.K.is leaving and only after that to agree on how they will cooperate in the future.
The EU is keen to stress its unity as it faces the wrenching departure of one of its biggest members, the first time a country has left the bloc in its 60-year history.
The UK must settle its Brexit bill and reach agreement on the rights of European Union citizens living in Britain before negotiations can move forward to trade talks, under guidelines set out in Brussels' approach to the upcoming discussions.
Negotiations on future trade relations between the United Kingdom and EU could begin as early as this autumn, European Council president Donald Tusk has indicated.
Ukip leader Paul Nuttall said that Brussels' wish to "play hardball with the reciprocal rights of individual citizens" was not in the interest of member states, and that as Brexit neared they would see the EU's "rigid approach", not the United Kingdom, as the main problem.
The EU leader also emphasized on the importance of supporting the peace process in Northern Ireland, saying that the EU will seek flexible and creative solutions aimed at avoiding a hard border between Northern Ireland and Ireland. Many EU officials believe it could be between two and five years.
The draft negotiating guidelines will set the tone for two years of tough talks to come.
The EU said meanwhile it would "prepare itself to be able to handle the situation also if the negotiations were to fail".
"It is clear both sides wish to approach these talks constructively and, as the prime minister said this week, wish to ensure a deep and special partnership between the United Kingdom and the European Union".
The letter outlines the EU's plan for talks with Britain to take place in three distinct phases.
"Strong ties reaching beyond the economy, and including security co-operation remain in our common interest", he said.
But time will be short as Mr Barnier has said that a draft deal is needed by October 2018 if it is to be ratified in time by the European parliament and member states.
This was interpreted as a threat - something the British government has since denied.
May's notice of the UK's intention to leave the bloc under Article 50 of the EU's Lisbon Treaty was hand-delivered to Tusk in Brussels by Tim Barrow, Britain's permanent representative to the EU.