Emmanuel Macron has said the UK will be trapped in a customs union after Brexit unless Downing Street offers European fishermen full access to British waters during the coming trade negotiations.

As the 27 EU heads of state and government took a decisive step towards sealing the terms of Britain’s split from Brussels after 45 years of membership, the French president laid down his red lines in the talks over the future relationship.

Macron said the EU’s demands on fisheries needed swift resolution after 29 March 2019 or the talks on a wider trade deal would fail leaving the UK in the “backstop” customs union envisioned in the withdrawal agreement.

Under the terms of Theresa May’s Brexit deal, to avoid a hard border on the island of Ireland, the UK will stay in a customs union after a 21-month transition period if a wider trade deal is not ready to come into force.

“We as 27 have a clear position on fair competition, on fish, on the subject of the EU’s regulatory autonomy, and that forms part of our lines for the future relationship talks,” Macron said.

“It is a lever, because it is in our mutual interest to have this future relationship. I can’t imagine that the desire of Theresa May or her supporters is to remain for the long term in a customs union, but to define a proper future relationship which resolves this problem.”

Play Video 1:14 'The best deal possible': EU leaders back Theresa May in Brussels – video

He added: “We will concentrate our efforts in order to obtain access to the British waters before the end of the transition period. And of course all of our fishermen will be protected.”

Earlier in the day, at a special Brexit summit in Brussels, the leaders of the 27 member states unanimously backed the terms of the voluminous draft withdrawal treaty, covering citizens’ rights, the £39bn divorce bill and the solution for the Irish border, along with a 26-page political declaration setting out the basis of the future relationship.

The European commission president Jean-Claude Juncker congratulated the member states on maintaining their unity during the 18 months of talks despite “pressure from the UK”, but his focus during an end-of-summit press conference was on sending a message to the UK parliament.

Juncker warned British MPs planning to send May back to Brussels by voting down her Brexit deal that it would take the EU just “seconds” to crush their hopes.

In an uncompromising and direct message to Westminster, the commission president claimed the bloc had made its final offer.

“I am totally convinced that this is the only deal possible,” Juncker said. “Those who think that by rejecting the deal that they would have a better deal will be disappointed in the first seconds after the rejection of this deal.”

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“I am inviting those who have to ratify this deal in the House of Commons to take this into consideration: this is the best deal possible for Britain, this is the best deal possible for Europe,” he added. “This is the only deal possible – the only deal possible.”

The European council president, Donald Tusk, conceded that ratification of the deal in the UK parliament would be “difficult”.

Such is the level of concern that a senior EU official said a number of leaders had wished May “good luck” during the hour-long meeting with the British prime minister.

It is understood that some in the room had also sought to discuss how the EU would respond to May’s deal being rejected, but that it had not been encouraged. “After this long, long process I really can’t see how we could ever go there,” the official said.

We don’t want to create the false impression a better deal can be negotiated. It can't. Leo Varadkar

The chancellor of Austria, Sebastian Kurz, said: “It is a take-it-or-leave-it situation and the British negotiators know this and hopefully also the British delegates and now it is up to them to make a decision,” he said.

Leo Varadkar, the Irish prime minister, said leaders had made a “conscious decision” not to discuss an alternative to the deal, adding it was a case of “this deal or no deal”. “We don’t want to create the false impression to the UK that a better deal can be negotiated,” he said. “It can’t.”

Macron and the German chancellor, Angela Merkel, offered more nuanced responses when asked what the EU would do should the British parliament vote the deal down. Merkel said it was “very difficult” to respond to “speculations”.

“If that were the case it would first of all be for Great Britain to make proposals, but we should be prepared for any situation,” Macron said, referring to France’s “no deal” draft legislation.

Asked if there was any divergence in views within the EU, an official said Juncker’s comments had been a “very realistic assessment that after this tortuous and long negotiation that what is on the table is the only possible deal given the positions of the UK”, in a possible indication that only a major shift in the British government’s red lines could offer hope of further negotiations.

On endorsing the deal, the EU had issued a statement committing to build “as close as possible a partnership” with the UK after Brexit, while warning that they would be “permanently seized” in future negotiations by the principle that countries outside the bloc cannot enjoy the same rights as those within.

Brussels has rejected proposals thrashed out this summer at Chequers, the prime minister’s country retreat, to achieve “frictionless trade” in goods after Brexit.

Should the withdrawal agreement be ratified in Westminster and the European parliament, it is agreed that the UK will stay in the single market and customs union, without representation in any decision-making institutions, for a 21-month transition period following withdrawal on 29 March 2019.

An extension of that period of “up to one or two years” is foreseen should the negotiations over the future relationship not be completed by the end of 2020.

EU officials said they believed the meaningful vote on the deal would be on 10 or 11 December, just before the next scheduled meeting of EU leaders in Brussels.

The agreement had appeared at risk in the days leading up to the summit, when Spain’s prime minister, Pedro Sánchez, threatened to withhold support unless Britain conceded that the British overseas territory of Gibraltar, over which Spain has a long-running territorial claim, would be covered by a future trade deal only with Madrid’s consent.

The British ambassador to the EU, Sir Tim Barrow, gave that commitment in a letter on Saturday afternoon, prompting outrage across the political spectrum by what was described as a “betrayal” of Gibraltar.

On Sunday, Sánchez, who has vowed to open talks on joint sovereignty of the disputed territory, described the concession as “a very significant political achievement”.

“It places us in a position of strength in a central theme,” he said. “The idea of the government of Spain is to solve once and for all a conflict that has existed for more than 300 years.”