BRUSSELS (Reuters) - The European Union would have to see a breakthrough on Brexit within a week if its leaders are to endorse any deal with Britain in November, several official and diplomatic sources in Brussels said on Wednesday.

An EU summit tentatively scheduled for Nov. 17-18, is no longer on the cards due to a lack of the “decisive progress” towards a final deal on the Irish border, the sources told Reuters on a day when EU summit chair Donald Tusk spoke to Prime Minister Theresa May to discuss how to proceed.

An update of EU national envoys in Brussels from the bloc’s negotiators was delayed from Wednesday to Friday - an indication that EU negotiator Michel Barnier’s team is still waiting for a response from London on proposed compromises on a mechanism to avoid Brexit disrupting the sensitive Irish border.

Some diplomats have speculated that May could use her visit to Belgium for World War One commemorations on Friday to make a dash to Brussels to lock in an agreement, if she gets her cabinet behind it by then.

Barnier needs to see “decisive progress” in the talks to recommend to Tusk that a summit should be called.

Irish Prime Minister Leo Varadkar said on Wednesday that as the days went by a summit in November was getting less likely.

He noted that the EU will hold a regular summit on Dec. 13-14, though EU officials say it might still be worthwhile holding a special summit earlier in December.

For now, EU sources said the bloc was waiting for a response from London on whether it would accept the proposed compromise solutions worked out between British and EU negotiators on the emergency fix to ensure the Irish border stays open.

A meeting of May’s cabinet on Brexit on Tuesday had not produced enough clarity, they said.

IRISH BORDER

One thing they are looking at is linking a possible extension of a status-quo transition period after Brexit next March with the backstop to make the latter more palatable to London, the sources said.

The post-Brexit transition period is due to end at the end of 2020. Should no deal on EU-UK ties be ready to regulate their relationship and maintain the invisible border between EU state Ireland and Britain’s province of Northern Ireland from then on, the border backstop would kick in.

In an earlier compromise, the EU negotiators tentatively agreed to have a joint customs zone with all of the United Kingdom in such a scenario, complemented by keeping Northern Ireland inside the full EU customs union to prevent extensive checks on what will become the only EU-UK land border.

London opposes the specific Northern Ireland element, saying it would pull the province away from mainland Britain. If the sides agree an extension of the transition period, it would delay in time any prospect of the backstop being activated.

It would also give the EU and UK more time to negotiate the all-UK customs arrangement, which has been sought by London but which the bloc says would likely take more time to work out.

As well as a review mechanism for the backstop - which London wants to be able to get out of and the EU insists must be permanently available or else is no real insurance policy - a decision mechanism for extending the transition period is under discussion.

The EU rejects Britain’s push to be able to walk away from these unilaterally, sources in Brussels said. Barnier’s team is due to update EU ministers from the 27 member states on Monday.