European Union leaders may also talk about how to manage a transition, after Britain leaves in 2019, to a new relationship likely to take many more years to finalize.
Brexit has offered the European Union a fresh chance at unity after years of bitter internal divisions over the euro and migration, although many still fear they could fall out during the talks.
"There will inevitably be a price and a cost for Britain, it's the choice they made", Mr Hollande said as he arrived at a Brussels summit.
The European Council president said reciprocal guarantees, which would also provide security for Brits living in Europe, were key issues to be addressed before talks turn to trade deals.
Asked if he agreed with Mrs Merkel's analysis, EU Commission president Jean-Claude Juncker told reporters in Brussels: "That's my impression, yes".
President Tusk tweeted the outcome of their discussions, saying: "Guidelines adopted unanimously. This must be the number one priority", Tusk said. Britain will be reminded in the coming weeks to abide by the terms of the 1998 Good Friday Agreement.
A deal must also be agreed to avoid a hard border between the Republic of Ireland and Northern Ireland.
Also contentious will be which countries scoop the prizes of hosting the European Medicines Agency (EMA) and the European Banking Authority (EBA), the two EU agencies set to be moved from London.
The 27 leaders, who are meeting at a summit in Brussels, took less than 15 minutes to endorse draft guidelines which were issued on 31 March.
The first of the key divorce issues the European Union guidelines say Britain must resolve is the fate of three million European Union citizens living in Britain and one million Britons on the continent, and what happens to their rights to work and claim benefits overseas. This is estimated at around €40 billion (S$60.9 billion) to €60 billion, and mainly covers financial commitments made by the bloc while Britain was a member. The negotiations would begin only after Britain's general election on June 8, she said.
Luxembourg Prime Minister Xavier Bettel put Theresa May's decision down to internal, not global problems.
Britain's right-wing press fulminated last month against EU plans to spell out that Spain, which claims sovereignty over Gibraltar, should have a veto over applying any future EU-UK free trade deal to the tiny British enclave.
"Over the past weeks we have repeatedly heard from our British friends, also during my visit in London, that they are ready to agree on this issue quickly but I would like to state very clearly that we need real guarantees for our people to live, work and study in the United Kingdom and the same goes for the Brits", Mr Tusk said.
"I'm very clear that at the end of the negotiations we need to be clear not just about the Brexit arrangement, the exit, how we withdraw, but also what our future relationship is going to be".
Both the United Kingdom and Irish governments want to maintain the soft border, but have struggled to define how such an arrangement could work after the introduction of what would be Britain's only land border with the EU.
The united front comes hot on the heels of a war of words between German Chancellor Angela Merkel and British Prime Minister Theresa May over the negotiations.
The deadline for completing the negotiations is 29 March 2019.