Brexit negotiators have yet to seal a deal on Britain's divorce terms, officials said, after a flurry of meetings in Brussels on Sunday to try to secure a draft agreement before Prime Minister Theresa May arrives for a summit this week.
Sabine Weyand, Barnier's deputy, told ambassadors it was clear from Sunday's talks that for domestic reasons the United Kingdom needed more time before it could make concessions, according to a diplomat in the meeting.
"And delivering this is vital for our security too because economic relations sit at the heart of all our partnerships including the vital security alliances we have with our friends in Europe".
Irish Prime Minister Leo Varadkar said Monday that "I figure November or December is the best opportunity for a deal".
In a strongly-worded article in the Belfast Telegraph, Mrs Foster warned against the EU's backstop proposal and also stressed that she would not accept any measure that resulted in extra checks for goods travelling between Great Britain and Northern Ireland.
Sunday's talks ended without a breakthrough on the crucial issue of trade to and from Northern Ireland, which has emerged as a possible deal-breaker and even a threat to May's leadership.
Even if she reaches a withdrawal agreement, she will struggle to get it through parliament and may find opposition from the small Northern Irish party which props up here minority government to other legislation such as the budget.
The potential extension of the transition period, which could see the United Kingdom stay in the EU single market and customs union, accept continued free movement and make further payments to Brussels, is being considered as intensive negotiations continue ahead of the European Council summit.
EU Council president and summit host Donald Tusk searched for a positive outlook.
Health minister Matt Hancock suggested the backstop could be temporary without such a date, an argument that may fall flat for some eurosceptic lawmakers who are calling for May to "chuck Chequers", her Brexit plan named after her country residence.
And she lashed the EU's "backstop to a backstop" to the thorny issue of the Irish border, which would leave Northern Ireland tied to the the bloc's customs and some single market rules if no free trade deal is done in time.
Insisting that a Brexit divorce deal was "achievable", May said the border dispute should not "derail the prospects of a good deal and leave us with the no-deal outcome that no one wants".
Britain is leaving the European Union on March 29 and the possibility of there being no deal on trade and future relations is worrying businesses and politicians.
After Brexit, the now invisible frontier between Northern Ireland and Ireland will be the U.K.'s only land border with an European Union nation. He reportedly said he was prepared for all scenarios.
Cabinet minister Matt Hancock sought to play down speculation that some of his colleagues might quit over the Brexit plans, but was unable to say whether a fixed deadline for any customs arrangement would be written into a deal with Brussels.
He said that a backstop "cannot be time-limited".
Britain has long said it wants an agreement on a future relationship with the European Union, which London sees as including a common rule book for manufactured and agricultural products, something that would negate any need for a backstop plan for Ireland.
"There remain a number of unresolved issues relating to the backstop", a British government spokesman said on Sunday.
"We don't really know yet what really happened and what the real problem was", the diplomat confessed, adding that they expected "transparency" and updates at a meeting between the 27 countries' Europe ministers and Michel Barnier in Brussels on Tuesday.