Brexit may become “even more exciting” when Boris Johnson is in Downing Street but the deal will not change, Donald Tusk has said, as the EU readied itself for a new British prime minister.

An offer to listen to the ideas of whoever replaces Theresa May came with a warning from the European council president and fellow leaders that the withdrawal agreement was final.

“Maybe the process of Brexit will be even more exciting than before because of some personnel decisions in London, but nothing has changed when it comes to our position,” he said.

Johnson and Jeremy Hunt will spend the next four weeks lobbying Conservative members for their votes, with both men claiming they will be able to renegotiate the Brexit agreement.

But Tusk said that during a 12-minute discussion on the latest Brexit developments at a Brussels summit, the EU leaders were united in rejecting any further talks on the withdrawal agreement containing the contentious Irish backstop.

“We emphasised again that from our point of view the exit agreement has been finalised in negotiations but we will get back to this after the election of a new prime minister,” the German chancellor, Angela Merkel, later told reporters.

Tusk, whose job is to chair the leaders’ summits, went on to complain that it was a political reality that the UK had been “wasting” the extra time granted in April. He also failed to deny that he had been thinking of Johnson earlier this year when he claimed Brexiters without a plan deserved “a special place in hell”. “I think you understood what I thought using this metaphor,” he said.

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The French president, Emmanuel Macron, gave a political obituary to the outgoing British prime minister that was designed to be a warning to Johnson but which may not have been comfortable listening for May, who has been accused of being soft in the talks.

After speaking his regret at her departure, and the Brexit process, Macron told reporters at the end of the summit: “Theresa May, all the way through the negotiations she’s been incredibly loyal and respectful, she never stood in the way of Europe. We sometimes disagreed, but she never took the discussions on the future or Europe hostage.

“I do not know who will come next, and I certainly do not want to interfere in British life, but I hope it will be the same sense of respect and seriousness that she had. And all that truly matters is we’re able to reach a final decision.”

The Irish prime minister, Leo Varadkar, said he hoped for an “early meeting” with the new incumbent in Downing Street, who is due to be chosen by the Tory membership in the last week of July. He said: “I am conscious of the fact that notwithstanding their support for Brexit, their strong support of Brexit, both of the two people that are now going forward to the members of the British Conservative party actually voted for the withdrawal agreement and they did so only a few weeks ago, and I think that is something worth bearing in mind.”

The EU remains open to reviewing the political declaration on the future relationship, a legally non-binding adjunct to the withdrawal agreement that could be redrafted to offer deeper economic ties with the bloc, including a customs union.

But Varadkar said that a withdrawal agreement without the Irish backstop would be tantamount to a no-deal Brexit as it would not offer any assurances against a hard border.

Under the backstop, Northern Ireland would in effect stay in the single market, and the UK would share a customs territory, leaving it unable to pursue an independent trade policy.

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The Dutch prime minister, Mark Rutte, echoed the claims made by Varadkar on the first day of the summit that the EU was hostile to another extension of the UK’s membership beyond the end of October unless the government’s red lines changed or a general election or second referendum were called.

Rutte said: “I believe on October 31 it ends, except if you have elections or a new referendum or you have a new prime minister going to Brussels saying, ‘I want to discuss the red lines,’ and then you could have a look at the political declaration.

“But, only in that case. We’re not going to renegotiate if nothing changes because that would go around in the same circles.”

May will take part in one final summit a week on Sunday when the EU’s leaders are gathering again in Brussels after failing to agree on candidates to replace Tusk and the European commission president, Jean-Claude Juncker.

Other posts to fill in the coming weeks are presidents of the European central bank and the European parliament and the EU’s high representative for foreign affairs.