Michel Barnier, EU's chief negotiator, has emphasised the need to be flexible while also warning that the "clock is ticking" if an agreement is to be reached by the time the United Kingdom is scheduled to leave at the end of March 2019.
Stating that Britain is "unconditionally committed" to maintaining European security, the government said it wanted to contribute military assets to EU operations after it leaves the bloc and may offer to continue exchanging classified information.
European Commission President Jean-Claude Juncker has warned that Britain will regret its decision to leave the European Union and that the bloc will "move on" after London's departure.
"Nonetheless, we have to respect the will of the British people".
A spokesperson said "both sides" had agreed to push back the date to give negotiators "flexibility" to make progress at the current stage.
The office of Michel Barnier, the EU's chief Brexit negotiator, declined to comment. "However, the European Parliament has made it clear that, whatever the outcome of the negotiations on the future European Union-United Kingdom relationship, they can not involve any trade-off between internal and external security, on the one hand, and the future economic relationship, on the other hand".
Turkey's own long-stalled membership bid was ruled out "for the forseeable future" as a result of President Recep Tayyip Erdogan's crackdown following a coup attempt previous year, he said.
Ireland's government has called on Britain to stay in the EU's customs union to maintain the now invisible border between it and the British province of Northern Ireland, an outcome that would also limit disruption for exporters.
The EU also remains deeply divided by the migration crisis, with a fresh spike in arrivals in Italy from Libya adding to rifts over how to handle the biggest influx of refugees and migrants since World War II.
Juncker told Politico Europe newspaper last week that "while I am not an integration fanatic, I am very much in favour of deepening the European Union and at the same time respecting to the fullest extent justified, national interests". He also called for trade pacts with Australia and New Zealand.
FILE PHOTO - EU and British flags fly outside the European Commission building in London, Britain August 12, 2017.