"I believe there is a new dynamic now in the negotiations", May said as she left the Brussels summit.
At a summit of EU leaders in Brussels, European Council President Donald Tusk tweeted that the "EU 27 has adopted guidelines for the future EU-UK relations".
"All a transition period will do is kick the can down the road on the questions of the Irish border, future trade, security and all the other major decisions that Brexit has thrown up about the future of our country", the supporter of the pro-Remain Open Britain campaign group said.
British Prime Minister Theresa May (L) speaks with Slovenia's Prime minister Miro Cerar during a breakfast meeting on the second day of a summit of European Union (EU) leaders at the European Council headquarter in Bruxelles, on March 23, 2018. Michel Barnier said the transition period will end on December 31, 2020 and the United Kingdom will obey European Union rules and regulations until that date without any participation in the EU's decision-making policies. "I believe we're approaching this with a spirit of cooperation, with a spirit of opportunity for the future as well and we will now be sitting down and determining those workable solutions for Northern Ireland but also our future security partnership and economic partnership".
The 27 EU states, without Britain, approved a clause a year ago that "after the United Kingdom leaves the Union, no agreement between the EU and the United Kingdom may apply to the territory of Gibraltar without the agreement between the Kingdom of Spain and the United Kingdom".
The EU document also envisages "ambitious provisions on movement of natural persons, based on full reciprocity and non-discrimination among member states".More news: Gun Violence Awareness 'March For Our Lives' In Sartell Saturday
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This will cover "trade and economic cooperation as well as other areas, in particular the fight against terrorism and global crime, as well as security, defense and foreign policy", according to the document.
However, he said a lot of hard work still had to be done to allow a smooth transition in terms of the food industry's trade with the European Union, at the same time as opening new export markets.
Britain has said it is leaving the EU's single market and customs union, but the prospect of a "hard" border in Ireland has sparked concerns about the fragile peace on the island.
Chief EU negotiator Michel Barnier, who kissed Mrs May's hand as they met in the entrance lobby of the Europa building in Brussels, said Friday marked a "decisive" moment in "this hard and extraordinary negotiation".
"It is not a case of when they are winning we are losing or when they are losing we are winning". "Eventually we're going to fall off".