The leading EU Brexit negotiator rejected Boris Johnson's request for the removal of the Irish backstop.
Michel Barnier stated that the reverse stop ̵
Mr Johnson had earlier told the EU that the agreement should be canceled if Brexit was to be avoided without a deal.
Meanwhile, the prime minister told the rebel tories that they were facing a "major choice" to find siding with him or Jeremy Corbyn.
His comments come as some MPs who oppose Brexit without a deal – including conservatives – plan to take action in parliament next week.
- Why is the Irish border blocking Brexit?
- And what is it
The UK is due to leave the EU on 31 October, with or without a deal.
The back stop is part of a withdrawal agreement negotiated between Brussels and former Prime Minister Theresa May, which has been rejected by Parliament three times.
If implemented, Northern Ireland would remain aligned with certain EU single market rules if the UK and EU did not agree on a post-Brexit trade deal.
Mr Johnson said there was some movement from the EU as he sought to mediate a new negotiation and removal of the agreement, which he described as 'undemocratic'.
However, writing in the Sunday Telegraph, Mr Barnier said: “We have had intensive discussions with the EU Member States on the need to guarantee the integrity of the EU single market while keeping this border fully open.
"In this sense, the reverse stop is the maximum amount of flexibility the EU can offer to a non-member country."
Mr Barnier also said he was "not optimistic" about avoiding Brexit without a deal, but "we must all continue to work decisively."
He added: "The EU is ready to explore any avenues the UK Government can present that are compatible with the withdrawal agreement."
The EU could not stop the UK from leaving without a deal, he said, but he "will not understand the logic behind this choice" because "we will still have to solve the same problems after October 31".
Mr Johnson says he wants to leave the EU on October 31 with an agreement, but it is 'do or die' and is ready to leave without one, instead of missing the deadline.
This position prompted a number of opposition MPs to come together to try to block a possible deal.
Deputies opposed to unsuccessful Brexit are expected to try to gain control of parliamentary agenda this week to pass legislation that,
But Mr Johnson warned Tory lawmakers considering considering opposition parties groups that risk plunging the country into chaos.
In an interview with the Sunday Times, he said: "Just telling everyone in the country, including everyone in Parliament, the main choice is this: Will you face Jeremy Corbin and those who want to repeal the referendum?
"Are you going to go to the side with those who want to rub the Democrats' atic judgment on the people – and plunge that country into chaos.
"Or will you stand on the side of those of us who want to embark, give the mandate to the people, and focus with absolute, laser precision on the internal agenda? "
His comments come after Sun announced that No. 10 will not stop any Tory MP voting to block the unsuccessful Brexit
The report prompts former Chancellor Philip Hammond to say that it would be "shockingly hypocritical" for the government to fire Conservative MPs who rebel over their Brexit plans as eight cabinet members oppose party whip this year by voting with
Demonstrations were held in the United Kingdom on Saturday in response to Mr Johnson's plans to suspend Parliament in preparation for Brexit.
The prime minister, who announced this move on Wedne sday , said it would allow the government to pass new legislation.
But the decision provoked angry reaction from some Brexit politicians and opponents without a deal.
Labor leader Jeremy Corbin accused Mr Johnson of "breaking up and plundering our democracy" to force himself without reaching a deal, leaving MPs with too little time to pass legislation in parliament aimed at preventing such an outcome.
But Chancellor Sajid Javid defended the decision and added: "It is right because we are focusing on people's priorities.
If the progression happens as expected, Parliament will be closed for 23 working days.
Deputies must approve rest dates, but they cannot block the prophecy.