The European council president has said the UK is prepared to either cancel Brexit or to depart with no deal if MPs vote down the settlement secured by Theresa May with Brussels.

Speaking at the G20 summit in Argentina, Donald Tusk said there was no other deal on offer and the only options were to remain in the EU or leave with no deal.

“The European Union has just agreed an orderly divorce with the United Kingdom,” he said. “A few days before the vote in the House of Commons it is becoming more and more clear that this deal is the best possible, in fact the only possible one.

“If this deal is rejected in the Commons, we are left with, as was already stressed a few weeks ago by prime minister May, an alternative. No deal or no Brexit at all. I want to reassure you that the EU is prepared for every scenario.”

Tusk’s comments came as EU officials also dismissed May’s suggestion to MPs on Thursday that extending Brexit talks could result in renegotiating the deal, as Brussels awaits the next twist in the British political drama.

EU officials were perplexed by the prime minister’s comments to the House of Commons liaison committee on Thursday that extending article 50 would mean “you are then on the business of renegotiating the deal”. Although May went on to say that the current deal was the only one on offer, her remarks raised eyebrows in Brussels.

One view is that she is scaremongering in the hope of getting her MPs to walk through the yes lobby, when parliament votes on the deal on 11 December.

The EU regards the 585-page withdrawal treaty, painstakingly negotiated over 17 months, as a closed book. Officials say only the date of entry into force – 30 March 2019 – would be changed if article 50 was extended, and not a comma more.

Article 50 could only be prolonged if the UK made a big change to its Brexit position, such as dropping the demand for an independent trade policy to join the customs union, or requesting membership of the single market, through the European Economic Area.

“We could potentially reopen article 50, but not within the current frame, not within the current redlines,” one diplomat said. “There is just very little to talk about anymore – you could change the font.”

Liam Fox hits back at critics of Brexit deal as May rules out plan B - Politics live Read more

Speaking to the BBC on Friday in Buenos Aires, May appeared to row back and said there would be no renegotiation. “The EU have made clear that this is the deal. This is the deal that is on the table,” she told the BBC.

“So when MPs come to vote, they need to think about the need for us to deliver Brexit for people, to do that in a way that protects their constituents jobs and protects their futures. The deal that does that is the deal that I have negotiated, the deal that is on the table.”

EU leaders, including May, will meet for a long-planned summit in mid-December, two days after the Commons vote. Officials in charge of preparations say there is no fixed plan on Brexit, because the UK political process is fraught with uncertainty. “The vote is on the 11th. We are not even sure if, on the 13th, [May] will be here. So, no, there won’t be anything substantial,” one diplomat said.

In the event of a no-vote, some think that EU27 leaders could hold a separate meeting without the British prime minister “for optics”, where they would announce a step up in no-deal planning.

Diplomats from the 27 member states have taken part in a day and a half of no-deal planning seminars this week, covering citizens’ rights, aviation, road transport, data protection and consumer protection.

While Brussels is paying close attention to the UK political debate, diplomats are reluctant to speculate on precisely how the bloc would respond to the myriad of scenarios swirling around Westminster, from no deal, to a second referendum, to the “Norway-plus” plan that would keep the UK in the single market and customs union.

Brexit TV debate: Theresa May says she will only face Corbyn Read more

In line with its strategy throughout the talks, the EU would ask the UK to spell out what it wants, if parliament rejects the plan. “First of all it would be for the UK to decide what to do,” a senior EU diplomat said.

The Norway-plus plan is seen as fraught with problems, not least securing the agreement of Norway, Switzerland, Iceland and Liechtenstein – the members of the European free trade area that some British MPs hope to join. “This is a very hypothetical question and I don’t see how we could get it together in the time available,” the diplomat said.

EU leaders have insisted there can be no renegotiation of the Brexit deal, a message reiterated by the EU’s chief negotiator, Michel Barnier, on Thursday. “The orderly withdrawal treaty that is on the table … is the only possible deal,” he told a small number of MEPs in a largely empty debating chamber.