This could force a dialogue on Gibraltar in which Madrid could have the upper hand.
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".
The EU's draft letter states that Britain's exit bill will be a once only payment.
He said: "Starting parallel talks on all issues at the same time as suggested by some in the United Kingdom will not happen".
Resolving the status of British and other European citizens living in each other's countries will be an early priority.
Mr Juncker and European Parliament president Antonio Tajani have backed the Irish case to safeguard the Belfast Agreement and the wishes of Dublin and London to avoid a hard Border. He added that "there is no one who wants to use citizens as hostages in this, no one".
Some felt Britain's departure letter hinted that London was threatening to end security cooperation unless it gets a good Brexit deal.
The United Kingdom formally began its exit from the bloc on Wednesday.
The proposals, presented to Parliament by Brexit Secretary David Davis, will also allow lawmakers to undertake the mammoth task of keeping, altering or ditching the thousands of European Union statutes that now apply to the U.K.as soon as the country formally withdraws.
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.More news: KFC to stop using chickens raised with human antibiotics
A Downing Street spokesman said: "These are draft guidelines and we look forward to beginning negotiations once they have been formally agreed by the 27 member states. For sure the European Union and Britain will continue to cooperate in this field, also through our cooperation with North Atlantic Treaty Organisation".
"It will then be for elected politicians in this country to make changes in the national interest", the Brexit ministry said.
Cooperation would in any case continue, she said.
He also insisted that Britain's commitment to European defense and security is "unconditional" and "not some bargaining chip in any negotiations" over Brexit.
Within hours of the result on June 23 previous year, in which 17.4 million people vote Leave in a 52 per cent to 48 per cent victory for Brexit campaigners, Mr Cameron resigned.
"The EU 27 does not and will not pursue a punitive approach".
The European Council had meant to publish the guidelines on its website straight away, but the BBC understands that two EU member states objected.
Speaking during a joint news conference with Maltese Prime Minister Joseph Muscat in the capital Valletta, Tusk outlined Brussels's guidelines during the withdrawal process of Britain from the bloc. "It will be a tough negotiation, but it will not be a war", he said.
Ireland's unique Brexit concerns will be flagged in the EU's draft blueprint for negotiations with Britain, to be circulated on Friday to member states.
Going into the two-year negotiation period triggered by Article 50 of the Lisbon Treaty (official process to leave the EU), the European Union would think of the people first, Tusk said.