Donald Tusk is pushing the European Union’s leaders to consider a long Brexit delay to allow the UK to rethink its goals in the negotiations as the Commons voted in favour of seeking an extension of article 50.

In an apparent shift in the EU’s red lines, the European council president suggested even before MPs had voted that a lengthy extension beyond 29 March could be granted simply to give Westminster time to recalibrate.

Officials have until now insisted that only calling a general election or second referendum could justify delaying Brexit beyond more than a few months.

“During my consultations ahead of [the EU leaders’ summit next week], I will appeal to the EU27 to be open to a long extension if the UK finds it necessary to rethink its Brexit strategy and build consensus around it,” Tusk tweeted.

However, a European commission spokesman expressed the concerns in Brussels over the impact of a long extension after MPs voted 413 to 202 to push back Brexit to at least 30 June.

“A request for an extension of article 50 requires the unanimous agreement of all 27 member states,” the spokesman said. “It will be for the European councilto consider such a request, giving priority to the need to ensure the functioning of the EU institutions and taking into account the reasons for and duration of a possible extension.”

Reacting to the vote, the Dutch prime minister, Mark Rutte, made clear his frustration. “What’s the point of whining on for months on end while we have been going around in circles for two years?” he said. “When Theresa May comes asking us for an extension, our response will be: ‘For what? To what end?”

However, delays of between a few weeks to as long as 21 months have been mooted in recent weeks, with the Irish deputy prime minister, Simon Coveney, being the latest on Thursday to suggest that a lengthy delay could be helpful despite the complications.

“If you have a long extension of article 50, that opens up the debate in a much broader way to the overall approach that the United Kingdom takes to Brexit. That may facilitate a fundamental rethink, it may not, we just don’t know,” Coveney said.

“If you have a long extension of, say 21 months to the end of 2020 – whatever the period would be – then Britain has a legal entitlement to have representation in the European parliament.”

Detlef Seif, the Brexit point person for the Christian Democrat party of the German chancellor, Angela Merkel, and that party’s leader, Annegret-Kramp Karrenbauer, told the Guardian the European commission could even relax its refusal to negotiate the terms of a future trade deal during such a 21-month extension in order to “rebuild trust”.

“I agree with [EU deputy chief negotiator] Sabine Weyand that the much-discussed technical solution for doing customs checks without the infrastructure of a hard border don’t yet exist,” said Seif. “But if Theresa May were to submit a declaration to Brussels in which she stated her intention to spend the next 21 months developing such technical solutions in order to avoid a hard border between Northern Ireland and the Republic, then that should suffice to grant an extension.”

To make an extension more attractive to hardliners in the Conservative party, Seif added that the EU should loosen its red lines around talks over the future relationship.

“It was right to rule out talks about the future relationship in the early stages of the negotiations, since the Brits showed very clearly that they would not have talked about their financial obligations otherwise. But now that we have a basic agreement in place, there should be space to talk about what Britain’s relationship with the EU will be in the future. We are getting to a place where both sides are expecting the worst of each other – it’s time to rebuild trust.”

Tusk’s move to back a potential lengthy extension will be seen by some as an attempt to help May scare Brexiters into supporting her deal or face a prolonged extra period of EU membership during which the risk of a softer Brexit or second referendum would rise.

But it also reflects the concern that a short extension of a few months would not resolve any of the issues in Westminster, making a no-deal scenario likely this summer.

The 27 heads of state and government are not agreed on the issue of an extension but would need to come to a unanimous view when they meet at the summit in Brussels on 21 and 22 March.

Guy Verhofstadt, the former Belgian prime minister, who is the European parliament’s Brexit coordinator, tweeted: “Under no circumstances an extension in the dark! Unless there is a clear majority in the House of Commons for something precise, there is no reason at all for the European council to agree on a prolongation.”

The prospect of a lengthy delay has been made more palatable after the UK’s advocate general in the European court of justice, Eleanor Sharpston, described claims that the European elections in May would be an “insuperable obstacle” to a lengthy extension as “oversimplified and fallacious”.

The prime minister, after yet another defeat in parliament, had told MPs in the Commons on Wednesday night that a long extension would probably require the election of British MEPs in May.

Sharpston said the UK could extend the mandates of its MEPs “who have already been democratically elected and who have been sitting in the current European parliament” or send “nominated MPs from the UK, rather than directly elected MEPs, to participate in the European parliament during that period”.