Emmanuel Macron has warned Boris Johnson that the UK must remain “loyal” to EU standards post-Brexit for British companies to maintain access to the European market.

In comments echoed by the German chancellor, Angela Merkel, the French president demanded continued regulatory harmonisation as the price for protecting the flow of trade, a demand that will be a cause of concern for the Conservative government.

Macron, who noted in an aside that his country would soon be the only nuclear power in the EU, told reporters: “If Boris Johnson wants a very ambitious trade deal, there has to be very ambitious regulatory convergence”, adding in English: “Be my guest.”

“We do not want them to be an unfair competitor,” Macron said. “My message to the UK is that the more loyal we are vis-a-vis each other, the closer relationship we can have.

“If the British prime minister and the British parliament want an ambitious trade deal, they know where the European standards are … The more they are attracted to reducing standards – on climate, social standards or anything else – the more they walk away from the European market, the more they will be away from us. The more ambitious the trade deal, the more we need regulatory harmonisation.”

The intervention from Macron came as the new commission president Ursula von der Leyen warned that the post-Brexit negotiations on the future relationship would drag on beyond 2020.

Johnson has said he will not extend the 11-month transition period, during which the UK will stay in the single market and customs union, but that this provides “ample” time for a comprehensive deal on the future relationship to be finalised.

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Von der Leyen appeared to directly contradict the prime minister by warning that certain parts of the deal would need to be prioritised, including trade in goods, fisheries and security cooperation, but that others, potentially including the arrangements for the financial services sector, would be left for later.

“We will have to prioritise,” Von der Leyen said at the end of a two-day leaders’ summit in Brussels. “We will put specific focus on those issues that were an economic cliff edge at the end of 2020 if they would not be done. Why is that? These are issues where we have neither a framework to fall back on nor the possibility to take unilateral contingency measures covering the period after 1 January 2021.”

Von der Leyen described the plan as a matter of “sequencing” adding that “there will be more emphasis on a certain ranking”.

The size of Johnson’s majority has been welcomed in Brussels, where the 27 heads of state and government have gathered for a two-day summit.

Charles Michel, the European council president, said he expected the withdrawal agreement to be passed through parliament within weeks to allow the UK to leave on 31 January.

“We expect as soon as possible a vote by the British parliament on the withdrawal agreement,” Michel said.

EU leaders believe the Conservative majority will give Johnson free rein to decide his negotiating objectives and compromise in the talks where necessary.

EU leaders are now waiting to see what incarnation of the prime minister will emerge now he is secure in Downing Street.

Quick guide How is Boris Johnson's Brexit deal different from Theresa May's? Show Hide The new Brexit deal is essentially the old Brexit deal with a new chapter on the protocol on Ireland and Northern Ireland and a few key tweaks to the political declaration. Here is a link to the full text. The backstop is replaced The backstop has essentially been replaced by a full stop whereby Northern Ireland remains aligned to the EU from the end of the transition period for at least four years. A change can only happen if it is voted on by the Stormont assembly. Consent Stormont will have a key role in future Brexit arrangements. And if there is cross-community support to remain aligned to the EU rather than the UK the consent will hold for eight years. The arrangements in this deal will automatically kick in for a mandated four years if there is a breakdown in trade talks, so it remains a “backstop” but with a permanent tinge. That four-year period will start at the end of December 2020. Two months before the end of the four-year period, that is October 2024, Stormont will be asked to vote on whether to remain aligned to the EU in ways outlined by this deal or not. Checks on border, ports and airports Under the deal, the UK and the EU are “underlining their firm commitment to no customs and regulatory checks or controls and related physical infrastructure at the border between Ireland and Northern Ireland”. Future trade deals The EU and the UK will aim for a zero-tariff deal with unlimited quotas. The entire UK, including Northern Ireland, will be free to sign trade deals. The line in the political declaration that “the United Kingdom will consider aligning with union rules in relevant areas” in any future trade talks has been ditched. Customs Northern Ireland will remain legally in the UK customs territory but practically in the EU customs unions. There will therefore be no customs checks on the border but tariffs will be payable on certain commercial goods. No customs duties will be payable on “personal property” being transited from Great Britain to Northern Ireland. That protects online shopping and all items bought for personal rather than commercial use. Customs duties will be payable on goods imported from the UK for commercial use unless it can be demonstrated that the goods remain in Northern Ireland or are for personal use, as above. A system of rebates will allow importers to be reimbursed. West/east trade The commitment to frictionless trade between Northern Ireland and Great Britain is restated. VAT EU law on VAT will apply in Northern Ireland. Single electricity market The island of Ireland is considering a single market for electricity so homes in Northern Ireland can get their energy from a supplier in Northern Ireland or the republic. There were fears this could be disrupted by Brexit. Under the Johnson deal, the provisions of union law remain so nothing will change. Level playing field This guarantees that the UK will remain in line with EU conventions on climate, environment and workers rights in a future trade agreement. Lisa O'Carroll Brexit correspondent

Johnson had initially approached the negotiations on the withdrawal agreement with bullish, uncompromising talk, but swiftly broke his red lines to concede to a customs border in the Irish Sea between Northern Ireland and Great Britain.

The Irish taoiseach, Leo Varadkar, hinted at private assurances from Johnson that he would sign up to EU standards in order to secure a trade deal.

“I’d like that to ensure we still have tariff-free and quota-free trade between Britain and the EU and to have a set of minimum standards so that nobody feels that there’s unfair competition or anyone’s trying to undercut them when it comes to labour rights, environmental protection and issues like that,” he said. “And [from] my conversations with Mr Johnson, I think he’s probably in a similar space, so it’s a case of now getting on with it.

“It’s a case of now getting on with it and I think the prospect is there to develop a new future economic partnership between the EU and UK, and one that’s going to be mighty and good for all of us.”

There is a debate among the EU27 and with the European commission on what might be possible to negotiate between 31 January, when the UK is expected to leave, and the end of the transition period 11 months later.

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The Dutch prime minister, Mark Rutte, conceded at the summit that it would be “quite a challenge given the short time available” to find agreement even on trade by the end of next year. “We have to discuss the precise approach to these negotiations,” he said.

The Danish prime minister, Mette Frederiksen, said her government would be pushing hard to maintain access to UK waters for Denmark’s fishing industry. “There is no doubt that fishing really matters,” she said.

Satisfaction that there will now be clarity from the UK over the next steps has, however, been tempered by sadness at the its imminent departure.

The Czech prime minister, Andrej Babiš, said: “It is obviously a giant success for Boris Johnson. He is a charismatic leader. He won and now they will leave, unfortunately. That is bad news for Europe.”