EU chief Brexit negotiator Michel Barnier said Saturday the bloc will not change the divorce deal agreed with Britain and that he is "not optimistic" of avoiding a no-deal outcome.

Writing in the Sunday Telegraph, Barnier said that the so-called "backstop" had to stay to protect the integrity of the EU's single market while ensuring an open border on the island of Ireland.

"I am not optimistic about avoiding a no-deal scenario, but we should all continue to work with determination," Barnier said, according to extracts of his article on the newspaper's front page.

"The backstop is the maximum amount of flexibility that the EU can offer to a non-member state."

British Prime Minister Boris Johnson, who took power last month, has called for the provision to be scrapped in order to reach a new deal ahead of the country's latest October 31 departure deadline.

Johnson has said he was encouraged in his bid to axe the backstop by recent meetings with German Chancellor Angela Merkel and French President Emmanuel Macron, who appeared to suggest an openness to alternatives.

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Meanwhile, demonstrators rallied on Saturday in cities across Britain against Johnson's controversial move to suspend parliament weeks before Brexit.  

The prime minister decided to prorogue parliament until October 14, two weeks before the Brexit deadline.  

The decision led to rising tensions between the government and parliament in a divisive country due to disagreements on how to leave the European Union.  

Johnson insists the move was to allow the government to hold a Queen's Speech and outline an "exciting" agenda for the future. 

But critics say his intention is to stop MPs from plotting against him to stop a no-deal Brexit. 

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The EU negotiator also said discussions about alternatives to the backstop could not begin until Britain had approved the divorce deal struck with Johnson's predecessor Theresa May late last year.

It was subsequently rejected three times by British MPs, leading Brexit to be delayed to October 31.

"We are ready to start this work immediately upon ratification of the withdrawal agreement, in parallel to finally creating clarity on our future relationship," Barnier stated.

He added that if Britain left the EU without a deal it would be London's decision.

"Many people in the UK understand that and I would be surprised if they succumb to the idea that the EU is to blame for a difficult political situation in the UK."

Writing in the same newspaper, Johnson's de facto deputy Michael Gove said that to remove the option of a no-deal Brexit on October 31 would "diminish" the "chances of securing changes" to the Brexit deal that could get it passed through parliament.

(With input from agencies)