With 149 days until Brexit day, time is running out to secure a deal that the British government wants to nail down this autumn, to allow time for the agreement to gain assent from parliament and the European parliament.

EU diplomats meeting on Tuesday agreed to hold a series of no-deal planning seminars in November, covering citizens’ rights, aviation, ground transport, customs, border controls and financial services.

Senior British and EU officials are in constant contact and neither side has given up on holding a special Brexit summit in mid-November to strike a deal. With British politics focused on the budget, EU diplomats do not expect talks between the EU chief negotiator, Michel Barnier, and the Brexit secretary, Dominic Raab, to resume this week.

Some European officials see a growing risk of sliding into an accidental no-deal, if the British parliament votes down any agreement the embattled prime minister strikes with the EU.

Philippe Lamberts, a Belgian MEP, who sits on the European parliament’s Brexit steering group, said he had become more pessimistic about a Brexit deal, following Theresa May’s speech to parliament last week, where she again rejected the EU proposals on Ireland, which the EU sees as reneging on earlier agreements. “Since the British government started backtracking on its commitments in the joint report [of December 2017] I have become less optimistic about a deal being clinched,” he told the Guardian.

EU leaders decided they would not come to a November summit unless there was substantial progress in talks, which are deadlocked over a solution to avoid a hard border on the island of Ireland. Barnier told EU leaders earlier this month not to expect progress until December, which leaves very little time for UK parliamentary ratification.

A UK government spokesperson said: “We want to get a deal as soon as possible and agreed in the autumn. We will continue to work with the EU to make this happen.”

Talks remain stuck over the Irish backstop, an insurance plan that would keep Northern Ireland in the EU customs union and subject to many single market rules. May has said no prime minister could accept it and the government is searching for alternatives. While EU negotiators have been discussing an all-UK customs union and extension of the transition, EU diplomats insist these cannot be alternatives to an open-ended guarantee on the backstop.

Senior EU sources say little progress has been made since March on Ireland, with the British rejecting Barnier’s efforts to “de-dramatise” the issue by moving most customs checks away from the border, into company premises.

EU leaders are said to be confident of a deal, but some officials are less sanguine. Several sources have rubbished claims that the German chancellor, Angela Merkel, will ride to the rescue of the British by watering down the EU position. “Germany has not budged one inch,” a senior EU diplomat said. “The UK has been misreading the German position for years,” the source added, a reference to David Cameron’s misplaced hopes in Berlin on a series of issues.

Merkel’s decision to step down as leader of the CDU party, which is seen as weakening her authority, is unlikely to have any impact on Brexit. “The Germans are pretty laid back,” said one diplomat, who predicted it would not change anything.

British politicians campaigning for a second referendum, including the mayor of London, Sadiq Khan, and the Liberal Democrat leader, Vince Cable, urged Barnier last week to start planning for an extension of the two-year negotiating period, article 50. Khan said whatever deal May brings back would definitely be rejected by parliament.

Diplomatic sources say the EU has made no plans for prolonging article 50, while any extension beyond the European elections on 23 May 2019 is deemed legally impossible, unless the UK takes part in the vote.

The UK will automatically leave the EU on 29 March 2019 unless all 27 member states agree unanimously to a British request to extend article 50.

Lamberts argues that British remainers should accept leaving the EU and campaign for re-entry during the 21-month transition period.

He voiced confidence the EU would fast-track the UK’s re-admission. “This is a special case, where you cannot as EU27 demand full re-application,” he said. “If during the transition a popular majority or a political majority in the House of Commons or House of Lords come to the conclusion that the UK should exit the transition by stepping back into the EU I really do not see how, politically, the EU27 could say ‘no thanks’.”

The UK would already be “100% like a member”, because it applied all EU rules and would be paying into the EU budget, he said. “The only negotiations would be on which conditions the UK would rejoin and of course it would be a hard negotiation, but I do not see how it would fail.”

Many remain campaigners staunchly oppose this strategy because the UK would lose the rebate and have to renegotiate existing special privileges, such as opt-outs on joining the euro and justice and home affairs.

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