European Council president Donald Tusk said EU leaders must “remain united” as the 27 countries meet to ratify guidelines for Brexit negotiations with the UK.
Leaders are today meeting in Brussels to discuss a joint strategy for the negotiations without the UK present at the summit.
Arriving, Mr Tusk said he believed the shape of a future trading relationship can only be considered when progress is made on the terms of the UK's departure.
"We need to remain united as the EU 27," he said.
"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.
"As for now I feel strong support from all the EU institutions, including the European Parliament, as well as all the 27 members states.
"I know this is something unique, but I am confident that it will not change."
A draft negotiating position was outlined by Mr Tusk last month.
There are unlikely to be any radical departures in the final framework, with the EU 27 expected to remain firmly committed to a "phased" approach to negotiations.
Mr Tusk has insisted "significant progress" must be made on disentangling the UK from its ties and obligations to the EU before discussions can turn to the post-Brexit relations.
One of the key issues in the first phase are the size of the "divorce bill" the UK will need to stump up on departure - estimated by EU officials at around £50 billion.
And addressing uncertainty over the rights of EU citizens living in the UK and British expats residing on the continent is also on the agenda.
Addressing media in Brussels, Mr Tusk said: "We all want a close and strong future relationship with the UK - there is absolutely no question about that.
"But before discussing our future we have to sort out our past, and we will handle it with genuine care, but fairly.
"This I think is the only possible way to move forward.
"We also need solid guarantees for all citizens and their families who will be affected by Brexit on both sides.
"This must be the number one priority for the EU and the UK.
"And the Commission has already prepared a precise and detailed list of citizens' rights we want to protect."
Mr Tusk has also signalled a desire to resolve the thorny problem of the Irish border - and how to avoid customs and immigrant checkpoints on the politically sensitive frontier - before moving negotiations to the second stage.
Aside from dealing with the present day border, the European Council may also address its approach if the day came when the people of Northern Ireland voted to end partition and join a united Ireland.
EU leaders are ultimately expected to agree that, in such circumstances, Northern Ireland would automatically assume the EU membership already held by the Republic of Ireland, rather than having to reapply.
One of the more controversial elements of Mr Tusk's draft guidelines in March was a suggested veto for Spain on any future UK/EU agreements that involved Gibraltar.
Another recent issue of contention that could feature on Saturday is whether a free trade deal would include the financial services industry and, if it did, whether City of London institutions would still be bound by Brussels oversight.
Former Northern Ireland first minister Lord Trimble said there was no need to include a passage on the possibility of a united Ireland, saying it would only "stir up nationalist feeling".