Dominic Raab plans to resume negotiations in Brussels next week, as the government insists it has not given up hope of sealing a Brexit deal in October — despite an admission by the Cabinet Office minister, David Lidington, the timetable could slip.

The Brexit secretary has promised to approach the talks with “vim and vigour” and intends to spend more time in direct talks with the EU commission’s negotiator, Michel Barnier, than his predecessor, David Davis.

Raab said on Thursday he believed a withdrawal agreement between Britain and the EU27 was 80% complete and Downing Street sources said both sides were keen to complete the talks “as soon as possible”.

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But with the issue of the Northern Ireland border still unresolved, there is considerable work to do and the government has to contend with a vociferous campaign from a hardline group of its own MPs keen to see the prime minister “chuck Chequers”.

In the first public acknowledgment by the government that October no longer looks a realistic deadline, Lidington said the EU would have no problem scheduling an emergency meeting to finalise the deal and that a November deadline would be “manageable”.

His statement did not come as a surprise – insiders have long expected the official timetable to slip – but it means the EU and the UK government are now openly saying that a final withdrawal agreement will probably take longer than expected.

Both sides have been working on the assumption that the deal is supposed be agreed at the European council meeting starting on 18 October, one of the regular summits for EU leaders scheduled well in advance.

The UK is scheduled to leave the EU on 29 March 2019, so the withdrawal agreement needs to be settled some months before to allow the UK and European parliaments time to approve it.

Anand Menon, director of thinktank UK in a Changing EU, said it could work to the government’s advantage at home if the negotiations slipped closer to the deadline, because there would be no time for MPs to force the government to renegotiate. “The more there is the threat of a precipice, the harder it is to reject the deal,” he said.

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This week, Barnier said October was no longer an absolute deadline, but that talks must be wrapped up “certainly not later than the beginning of November”.

In an interview with BBC Radio 4’s Today programme, Lidington noted “with interest” what Barnier had said and that scheduling an emergency European council meeting for November, to allow EU leaders to clinch the deal then, would be straightforward.

“I was Europe minister for six years. I lived through enough emergency European council meetings to know that the European council can call additional meetings when it wants to,” said Lidington, who effectively serves as Theresa May’s deputy.

He added that both sides wanted an agreement “as quickly as possible … but if it slips beyond October into November, I think that is manageable”.

The consensus around November appears relatively new. May told another EU leader during the summer that she needed a deal by October in order to get the Brexit deal through parliament, according to a diplomatic source.

“For the EU side, we could still have something in November, or even in early December, but apparently for the UK side [December] would be a problem,” said the source.

Some EU officials are alarmed about talk of delay until December, because they fear an extended round of parliamentary ping-pong between the Commons and Lords could mean the UK runs out of time to ratify the agreement.

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Barnier said earlier this week that a deal would have to be reached “not later than the beginning of November”.

EU officials have long expected an emergency Brexit summit in November, because the Conservative party conference in late September is seen as closing down May’s room for manoeuvre, limiting the chances for a breakthrough at the planned October meeting.

The EU thinks that ratification by the European parliament could be done in eight weeks. However, time will also be needed for the final texts to be scrutinised by European legal linguists and translated into the EU’s 24 official languages. But EU officials believe their processes will be speedier than those of the UK parliament.

In his interview, Lidington defended Philip Hammond, the chancellor, who has been criticised by Tory Brexiters for releasing a letter to the Commons Treasury committee saying a no-deal Brexit could eventually increase government borrowing by £80bn a year.

The letter was published on Thursday afternoon, just hours after the government released a set of papers intended to show that, if the UK had to leave the EU with no deal, it would be able to cope.

Lidington said there was “nothing new” in the analysis. “This is provisional analysis that the treasury published back in January this year and I think all Philip was doing was simply referring back to that in response to a senior member of parliament,” he said.

When it was put to Lidington that the timing of the letter’s release seemed deliberate, he replied: “We are sending out letters to MPs as ministers practically every day of the week trying to meet deadlines that we set ourselves for giving people replies within a certain number of days, otherwise we get ticked off for being dilatory in responding to parliament.”