EU President Donald Tusk warned yesterday that the second phase of Brexit talks, on the bloc's future relationship with Britain, will be the "real test of our unity", ahead of the two-day summit in Brussels.

"I have no doubt that the real test of our unity will be the second phase of the Brexit talks," Tusk told reporters as he arrived for the summit.

The 27 nations staying in the EU have maintained a unified front throughout the six months of at times acrimonious talks with London so far, but Tusk warned that the next stage, covering a post-Brexit transition phase and future relations between Britain and the EU, would be harder.

Britain now faces a compressed timetable to pass all the laws it needs to get ready for Brexit, a spokesman for Prime Minister Theresa May said on Thursday after lawmakers defeated the government on proposed changes to its EU withdrawal legislation.

"This is a very specific amendment which would compress the time we have to pass secondary legislation under Clause 9 in time for exit day, when the withdrawal agreement comes into force, so we now need to consider if further changes are needed," the spokesman told reporters. He said the government had no plans to withdraw a separate proposal to write into law its 29 March 2019 European Union exit date.

European Commission chief Jean-Claude Juncker struck a deal with May on Brexit separation issues on December 8, saying Britain had made "sufficient progress" on its divorce bill, the fate of the Irish border, and protections for EU expats living in Britain.

The deal was a rare moment of triumph for the British prime minister, who has been struggling to assert her authority since losing her parliamentary majority in a disastrous snap election in June.

But Tusk warned earlier this week of a "furious race against time" to reach a deal on a future relationship following last week's agreement on how best to sever Britain's membership of the EU.

The EU guidelines that leaders will approve on Friday say they will start talks on the transition in January but will not begin discussions on trade until March as it needs more clarity on Britain's goals, while security and defense will be covered later next year.