British Prime Minister Boris Johnson headed for the United Nations in New York on Sunday to argue that post-Brexit Britain will be a dynamic world power taking the lead on tackling climate change and an unstable Middle East. But he and his country face some big hurdles.

British Prime Minister Boris Johnson boarding his plane at Heathrow Airport as he heads off for the annual United Nations General Assembly in New York, Sept 22, 2019. (Photo: VCG)

He is struggling to strike a withdrawal agreement with a skeptical European Union, and a looming verdict from Britain's Supreme Court could derail his Brexit plans.

Johnson is likely to be dogged by Brexit throughout his three-day trip to the General Assembly, the UN's annual gathering of world leaders.

More than three years after Britain voted to leave the EU, the departure date has been postponed twice, and the UK Parliament has repeatedly rejected the only divorce deal offered. The country is facing a chaotic exit on Oct 31 unless Johnson's government can, against the odds, secure a new agreement.

In search of a deal, Johnson is scheduled to hold talks at the UN with EU leaders, including European Council President Donald Tusk, French President Emmanuel Macron, German Chancellor Angela Merkel and Irish Prime Minister Leo Varadkar.

But many leaders of the 28-nation bloc mistrust Johnson, a brash Brexit champion who played a big role in persuading British voters in 2016 to opt to leave the EU. And they say Britain has not come up with workable ways to maintain an open border between EU member Ireland and the UK's Northern Ireland — the key sticking point in the dispute.

A senior British government spokesman said talks with the EU so far have been positive, but "there is an awful lot of work still to do".

"What is obviously encouraging is that all sides do want to try and achieve a deal," he said, speaking on condition of anonymity to discuss the government's thinking.

European Commission President Jean-Claude Juncker said a no-deal Brexit "would have catastrophic consequences," including customs posts and other barriers between Ireland and Northern Ireland. But he said it would be Britain's fault.

"We did not invent the Brexit," Juncker told Sky News in an interview broadcast Sunday. "We were never pleading in favor of any kind of Brexit. That's a British decision, and so it has to be dealt with in that way."

Johnson also has a meeting scheduled with President Donald Trump, who has called the British leader "a really good man" and claimed that some refer to Johnson as "Britain Trump".