Ex-Italian PM says refugee influx will be at a high next summer, so referendum should be held in 2017 to avoid linking two issues

It would be a mistake for the UK referendum on continued membership of the European Union to be held this summer at the height of an expected EU migration crisis, Italy’s former prime minister Enrico Letta has said.

He cautioned that linking the two issues would be terrible for the outcome and that it would be better if the government delayed the referendum until 2017.

Letta said: “The acceleration to a referendum in the summer will be at the very height of the storm of the refugee crisis at European level, so I think it would be wise to have the referendum next year when I hope the refugee crisis will be better. If not, the link between the two issues will be terrible.”

His remarks echo concerns in other European capitals that David Cameron’s desire to hold a referendum as quickly as possible may come up against the threat that the EU will be in the middle of a full-blown crisis over refugees and possibly rethinking fundamental issues such as internal border control. Letta called for border control police at EU level to supervise external borders.

Nicola Sturgeon says EU referendum in June would be a mistake Read more

The fear is that conflict between EU states over the refugee issue would highlight European disunity and could prove a wildcard in the referendum that would benefit the out camp.

In Germany and France there are growing concerns that the refugee crisis will not be solved this year because of the ongoing civil war in Syria.

The Syrian crisis does not look likely to be settled with peace talks that were due to have started on Monday but are struggling to get off the ground. As a result a further 1 million refugees are projected to try to enter the EU this year.

The issue of referendum timing and the refugee crisis also concerns Downing Street, but it has been assumed that if the UK cannot complete its negotiations in time for the 17 February EU council and a referendum in June, then a deal would be reached in March, making a referendum possible in late September.

There has been little planning for a 2017 referendum, and it is understood the idea would be examined only as a last resort.

Refugee crisis: Europe's Donald Trumps waiting in wings for EU to fail Read more

Letta was among a phalanx of senior European politicians, including two former prime ministers, who said the British renegotiation agenda was either completely impossible, self-defeating or, at points, crazy. In particular, Britain was warned that its plan to prevent non-UK citizens from receiving in-work benefits for four years could attack one of the key tenets of the union, since it threatened the principle of free movement of workers and would require a treaty change that other EU countries would not tolerate.

The criticisms came at a “war game” of Britain’s renegotiation organised by the thinktank Open Europe and attended by two former European prime ministers and nine former ministers. The simulation was designed to reveal the points of conflict between the UK and the EU in negotiations.

Representing Britain at the session, Sir Malcolm Rifkind, the former foreign secretary, said the issue of migration was critical to the talks since polls showed there would be a 9% swing to the out camp if Britain did not achieve something substantial on migration.

But the former Dutch employment minister Art Jan de Geus warned Rifkind there was a burning ring of fire around Europe, and the UK could not demand anything it wanted.

Ireland’s former taoiseach John Bruton said the rest of the EU wanted to know whether Britain felt emotionally connected to the EU, or whether it wanted to be part of the project in good times but not bad.

Facebook Twitter Pinterest Sir Malcolm Rifkind and the former Irish taoiseach John Bruton during the ‘EU war game’ event in London on Monday. Photograph: Claire Greenway/Getty Images

Karel De Gucht, a former EU trade commissioner, said it was possible to discriminate on social security but it would be “completely impossible” to discriminate between two people who were working. That would require a treaty change for which there was no political will.

He pointed out that it had been the British government that had first insisted on allowing free movement of Polish workers as soon as Poland joined the EU in the 1990s when all other countries had wanted a seven-year transition before allowing free movement.

Poland’s former deputy prime minister Leszek Balcerowicz said the UK’s plan for a four-year ban on benefits unjustly attacked the EU’s fundamental principles, adding: “If you infringe the freedom of the movement of workers why should we allow the free movement of capital. Then you set in motion a destructive spiral”.

France’s former Europe minister Noëlle Lenoir said France would not accept revision of the treaty, especially when it involved changes to the free market of labour, capital and entrepreneurs. “It would be the first time in Europe’s history that the free market principles are put in danger,” she said, adding that the proposal was dangerous and populist since it framed the issue of migrants as a threat to economic growth.

Rifkind pointed out that other EU countries discriminated against other nations in provision of housing or student grants, and insisted there was a will to find a solution in the negotiations.