Brussels - The European Union and Britain have not yet forged an agreement on how Britain will leave the bloc, Brexit negotiator Michel Barnier has told EU ministers.

"Intense negotiating efforts continue, but an agreement has not been reached yet," the ministers said following the briefing, in a statement by all EU member states except Britain.

Ultimatum to Brussels

The statement comes as British Prime Minister Theresa May faces growing pressure from eurosceptics in her country to issue an ultimatum to Brussels.

About 80 conservatives, joined by Northern Ireland's Democratic Unionist Party (DUP), have warned May they would vote against any Brexit proposal she sends to parliament unless she drops her acceptance of a temporary, last-resort backstop for the border between the Republic of Ireland and Northern Ireland.

May has accepted in principle the proposal of a backstop - keeping the border open regardless of what happens in other Brexit negotiations - if a permanent arrangement cannot be found by the end of a planned 21-month transition after Britain leaves the EU on March 29, 2019.

But conservatives, along with the DUP, oppose any outcome that could leave Northern Ireland under different customs arrangements from the rest of the Britain.

Preparations for every Brexit scenario

For its part, the DUP, whose 10 lawmakers are needed to keep May's minority government in power, published on Friday a letter to May that argued the EU was "clearly seeking a permanent backstop enshrined in international law."

A deal on the Irish border has been one of the thorniest issues as Brussels and London try to secure a divorce agreement that would prevent Britain from crashing out of the EU next year.

In the statement, EU member states have referenced this dispute as one of the "key issues" under discussion. They also noted that preparations for every Brexit scenario must continue.