* Johnson cites "great deal" towards withdrawal deal
* But "gaps and still difficulties" remain - Johnson
* UK "whole and entire" needs to diverge from EU -Johnson (Adds detail from EU sources, Labour vote)
By Kylie MacLellan
NEW YORK, Sept 23 (Reuters) - British Prime Minister Boris Johnson cautioned on Monday that there would be no Brexit breakthrough at talks with European leaders in New York as gaps remained, but said significant progress had been made on striking a deal.
Three years after the United Kingdom voted in a referendum to leave the European Union, hopes of an emerging deal to ease the transition were stoked when Johnson said that the shape of an accord was emerging, and European Commission President Juncker said an agreement was possible.
But the two sides remain split over London's desire to remove the Irish "backstop" - an insurance policy to prevent a return of border controls on the island of Ireland - from the divorce deal struck by Johnson's predecessor Theresa May.
EU sources said no proper alternative for the border between Northern Ireland, a British province, and Ireland that ensures the integrity of the EU single market and customs union has been proposed yet by London, so no breakthrough is on the cards.
Johnson, who has vowed to deliver Brexit on Oct. 31 with or without a deal, will meet EU leaders on the sidelines of the United Nations General Assembly in New York, including Germany's Angela Merkel and Irish Prime Minister Leo Varadkar.
He will also discuss progress on reaching a Brexit deal with European Council President Donald Tusk.
"I would caution you all not to think that this is going to be the moment," Johnson told reporters on the plane to New York. "I don't wish to elevate excessively the belief that there will be a New York breakthrough."
He said a "great deal" of progress had been made since he took office in July, citing what he described as EU leaders' acknowledgment that the Withdrawal Agreement they reached with May needed to be changed. But Johnson added that there were "clearly still gaps and still difficulties".
An EU official said last week talks were going nowhere.
"We don’t even know how to read what they are doing. If they are genuinely trying to open a negotiation, it would take them another 6-9 months to get to something. Or is it just tactical, and aimed at avoiding the blame?" the official said.
With the outcome of the marathon Brexit process still uncertain, the opposition Labour Party will vote to decide its strategy on Monday, leader Jeremy Corbyn heading for a showdown with the widely pro-EU party membership over whether Labour should endorse a policy of remaining in the bloc.