Nearly 5 million British and EU citizens could be stuck in limbo after a no-deal Brexit, a senior EU official has warned, who said it could take several years “to pick up the pieces” of the UK crashing out of the bloc.

Politicians on both sides of the Brexit divide have urged negotiators to ringfence the existing agreement on citizens’ rights if the UK leaves without a deal on 29 March.

The Brexit withdrawal agreement protects most of the rights of the 3.4 million EU citizens in the UK and the 1.3 million British people in the EU. But since MPs rejected the agreement by a crushing majority last month, these protections have been thrown into doubt.

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A senior EU official rejected suggestions the agreement on citizens’ rights could be easily salvaged from the wreckage of a no-deal Brexit.

“[No deal] will be a hard landing, it will cause considerable disruption,” the official told the Guardian. “There will be no quick fixes to pick up the pieces.”

The 585-page withdrawal agreement was negotiated under the EU’s article 50, an “exceptional” clause, the official said, which would no longer apply once the UK ceased to be a member. Any new agreement would have to be ratified by national parliaments, in line with EU standard practice when concluding international agreements that affect national policies.

“We are in a different legal scenario,” the EU official said. “Normally when we have a mixed agreement [where policy competences are shared between the EU institutions and member states] the whole agreement takes a couple of years to ratify.”

The current agreement seeks to preserve citizens’ rights to residency, employment, healthcare and pensions – all areas that are shared or “mixed” competences in this context. In contrast, issues such as customs and competition policy are exclusive EU competences, meaning agreements in these narrow areas do not require the approval of national lawmakers.

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The EU’s strict interpretation means the British government would be unable to ratify the withdrawal agreement after March, even if MPs had a change of heart. “Of course it can form the core of a new agreement, but there are no quick fixes here,” said the official.

While several European countries, including France, the Netherlands and Germany, have told UK nationals they can stay after Brexit, many questions around employment, pensions, healthcare and education remain unclear.

British MEPs last week urged the European commission to be more interventionist to guarantee “continuity rights” of British nationals who had chosen to live in the EU. In a similar gesture, the Tory coalition behind the Malthouse compromise has said the UK should unilaterally guarantee EU citizens’ rights.

Campaign groups reject the EU’s legal arguments that citizens’ rights cannot be ringfenced and are urging both sides to do so under article 50.

Maike Bohn, the co-founder of the3Million, which campaigns for EU citizens in the UK, said failure to reach an agreement before Brexit day would result in a “much more precarious” situation.

“Nearly five million citizens have lived in legal uncertainty ever since the referendum and now face further erosion of their rights in case of no deal,” she said. “This is the last opportunity to do the only thing that is morally right: to salvage what has been agreed on citizens’ rights through the article 50 procedure, before Brexit day.”

Facebook Twitter Pinterest The EU’s chief negotiator, Michel Barnier, has said there will be border checks in the case of a no-deal Brexit but has not specified where checks would take place. Photograph: Thierry Monasse/Getty Images

She said the EU had made “a political choice to avoid cherry-picking”, namely separate agreements on different issues that would give the UK more scope to break the unity of member states.

“But people are not cars, nor cherries to be picked and negotiations have officially concluded. There is a moral obligation to protect the rights for a finite group of five million people who got caught up in this messy divorce. And time is running out.”

Jane Golding, a lawyer who is the co-chair of the British in Europe group, said: “This is an unprecedented situation. Article 50 has never been applied before, and if there is the political will, a legal way can be found.”

The EU hopes to retrieve parts of the withdrawal agreement in the event of no deal, notably the Irish border backstop.

The EU’s chief negotiator, Michel Barnier, has said “there will be checks in the case of a no-deal Brexit” but has not specified where checks would take place. Both the EU and UK have pledged to avoid installations along the border that could become a target for paramilitary groups.

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EU officials hope the UK would eventually sign up to the backstop, even if there is no agreement by 29 March. The EU’s deputy Brexit negotiator, Sabine Weyand, has said the two sides would be “stuck with the same scenario” over the Irish border in the event of no deal. “So you could imagine a no-deal scenario with the backstop being discussed,” she said last week.

The UK will also be asked to keep up EU budget payments, totalling a net contribution of almost £10bn in 2019, in the event of no deal. Brussels has said all payments to UK-based researchers, farmers and other beneficiaries will stop from mid-April, unless the government keeps money flowing.