The 27 leaders meeting without British Prime Minister Theresa May will approve guidelines saying trade talks can only start once London agrees divorce terms on citizens rights, its exit bill and Northern Ireland.
EU President Donald Tusk insisted Britain would also benefit if unity boosted the chances of a deal, after May accused the other 27 countries of ganging up on London.
"We need to remain united as the EU 27," Tusk said as he arrived at the 27's first official Brexit summit since May triggered the Article 50 divorce process exactly one month ago.
"It is only then that we will be able to conclude the negotiations, which means that our unity is also in the UK's interest," the former Polish premier told reporters.
The call for a united front comes hot on the heels of a war of words between May and German Chancellor Angela Merkel, who said Britain should not have "illusions" about the talks.
German finance minister Wolfgang Schaeuble said in an interivew published Saturday that "Britain cannot have advantages that other countries do not after its departure. Nothing is free."
'Number one priority'
The EU 27 have considerably toughened the guidelines since Tusk first unveiled them a month ago, with Brussels also drawing up a detailed list of citizens rights.
"We also need solid guarantees for citizens and their families, who will be affected by Brexit on both sides. This must be number one priority for EU and the UK," Tusk said.
This referred to the fate of three million EU citizens living in Britain and one million Britons on the continent, with officials hoping for a resolution on their status after the divorce by the end of the year.
In a further move that will rile London the EU is also set to back automatic membership for Northern Ireland if it reunifies with Ireland, and call for Spain to have a say over any deal that affects Gibraltar.
The EU 27 will call for action to avoid a "hard border" between the Republic of Ireland and the British province of Northern Ireland, amid fears that Brexit could undermine the peace process.
They will also discuss for the first time the spoils of Brexit -- the relocation of EU medical and banking agencies that are currently based in London.
But they leave the criteria vague and so at Saturday's summit the leaders will have their first serious discussion on what exactly constitutes sufficient progress.
While the EU says citizens' rights is a priority, the most touchy issue of all is likely to be Britain's exit bill.
This is estimated at around 60 billion euros ($65 million), which mainly covers financial commitments made by the bloc while Britain was a member.
But the issue also risks causing divisions within the EU as they squabble over how to meet any shortfall.
Schaeuble said Germany would not pay any more. "The money in the EU budget must suffice. We must spend money more efficiently than we currently do."
'Line up to oppose us'
The unity call comes after years of bitter internal divisions on everything from the euro and migration to how to tackle growing euroscepticism.
May's decision to call a general election in Britain on June 8, in a bid to shore up her mandate and strengthen her negotiating position, has only stiffened their resolve.
Actual Brexit talks are not expected to begin until after the British election, although the EU is set to give an official mandate to its chief negotiator Michel Barnier on May 22.
The run-up to the summit was marked by bad tempered exchanges between Merkel, Europe's most powerful leader, and May.
After Merkel's "illusions" comment, May hit back saying that "our opponents are already seeking to disrupt those negotiations -- at the same time as 27 other European countries line up to oppose us."
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)