After May triggered Article 50 of the Lisbon Treaty on Wednesday (March 29), Britain now has two years to negotiate the terms and separate itself from the EU.
Another issue is securing the rights of both millions of mainly East European EU citizens in Britain and British people living in other member states.
The EU companies in Britain would need to be safeguarded against losing the protection of the 27-member bloc's law and regulation, he said.
Britain should not expect a "UK rebate" from the European Union in talks over its exit from the bloc, Germany's foreign minister said Thursday, in reference to a budget discount won by then-PM Margaret Thatcher.
"The talks which are about to start will be hard, complex and sometimes even confrontational".
But Liberal Democrat leader Tim Farron, who campaigned against Brexit, said the guidelines showed "the strength of the EU in these negotiations and the carelessness of the United Kingdom government in isolating themselves from our European allies".
Britain's exit from the European Union will not affect joint Hungarian and British interests, which are a guarantee of good relations between the two countries, Iain Lindsay, the UK's ambassador to Hungary, told a news conference on Thursday.
Britain's Brexit Secretary has denied that Prime Minister Theresa May is attempting to use security as a "bargaining chip" to secure a favourable exit deal.
The British government unveiled The Great Repeal Bill: White Paper, which explains how officials plan "to convert European Union law into domestic law". He said the level of progress would be decided by the 27 member states.
The talks will take place in two phases, the first covering withdrawal and the second covering a future relationship.
"In an increasingly unstable world, that collaboration is more, not less, important for us all", she wrote in the Irish Times.
"We are tough on one issue as the European Parliament, which is that we can never accept that you have a status outside the European Union more favourable than membership inside the European Union", he said.
With European Commission president Jean-Claude Juncker indicating that the EU is likely to seek around £50 billion from Britain to settle existing obligations, prior agreement on the bill would rob Prime Minister Theresa May of a powerful weapon to influence the outcome of trade negotiations.
And when it comes to the U.K.'s land border in Ireland, which is key to the Northern Ireland peace process, Tusk's draft guideline says that "flexible and imaginative solutions will be required". The EU guidelines make clear there can be no talks on the shape of any future relationship at this stage.
Setting out the guidelines, Tusk said it was "only fair" that the United Kingdom should honor the financial promises it made as part of the 28-member bloc.