Britain's Prime Minister Theresa May and European Council President Donald Tusk attend a bilateral meeting during the Eastern Partnership summit at the European Council Headquarters in Brussels, Belgium, November 24, 2017. REUTERS/Christian Hartmann

BRUSSELS (Reuters) - Britain has only 10 days left to deliver on all three areas of its divorce terms with the European Union if London wants to start talks on a transition period after Brexit and a future relationship, the chairman of EU leaders Donald Tusk said.

“We need to see progress from UK within 10 days on all issues, including on Ireland,” Tusk tweeted on Friday after a meeting with British Prime Minister Theresa May in Brussels.

“Sufficient progress in Brexit talks at December council is possible but still a huge challenge,” he said on Twitter.

An EU official said that May agreed in the one-hour discussions that Dec. 4 was the “absolute deadline” to allow the EU’s Brexit negotiator Michel Barnier to recommend moving onto the next stage on trade and future ties.

“Tusk presented the timeline ahead of the December European Council, with Dec. 4 as the absolute deadline for the UK to make additional efforts, allowing Barnier to be in a position to recommend sufficient progress,” the official said.

“May agreed to this timeframe,” the official said.

The official said Tusk had warned that if there was no progress within next 10 days, that would make moving forward impossible. The official said that the way Ireland’s border with Northern Ireland functioned after Britain leaves the EU in March 2019 was still an issue.

“The UK will need to give credible assurances as to how to avoid a hard border before Dec. 4, as it is still unclear how this can be done,” the official said.