European parliament president says he cannot guarantee that chamber will not veto reform terms agreed by EU leaders

David Cameron’s insistence that settlement terms defining a new deal for Britain in the EU must be immediately legally watertight and irreversible has suffered a setback after the president of the European parliament said he could not guarantee that.

The prime minister was in Brussels to meet Martin Schulz, the German social democrat who is the head of the parliament, as well as other leaders of the chamber.

Cameron sought guarantees that the parliament would not seek to unravel draft settlement terms being negotiated at a crucial EU summit on Thursday. Downing Street’s rush to the parliament reflected concerns that the chamber could be a loose cannon in the delicate renegotiations since changes in EU law to accommodate key UK demands on welfare curbs for EU immigrants will need to go through the parliament.

The meeting came as the president of the European council, Donald Tusk, said positions were hardening, there was a real risk of the EU breaking up. Leaders of Poland, Hungary, Slovakia and the Czech Republic were staging a mini-summit in Prague to hammer out a common position on the proposed British deal.

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While it would be highly unusual for the parliament to veto decisions taken by the EU’s 28 heads of government, the chamber’s role almost certainly means Cameron will only be able to effect the welfare changes much later than he hoped, and well after Britons have actually voted in the in/out referendum.

After their talks on Tuesday, Schulz said the parliament would not veto decisions taken by EU leaders, but emphasised the centrality of the parliamentary process.

“I can’t give a guarantee for the future of a legislation,” he said. “No government can go to a parliament and say: ‘Here is our proposal, can you guarantee a result?’”

The British had pressed for the parliament to issue a declaration this week stating that it would abide by the decisions taken at the summit. But it will not do this because it has to wait for the European commission to table detailed legislative changes before it can reach a verdict. The commission proposals can only be tabled once the result of the UK referendum is known.

If the British vote to leave the EU, the entire exercise is redundant. If they vote to remain, Britain will remain in the EU regardless of what happens in the parliament.

But the procedural argument punctures some of Cameron’s claims before the summit.

Shulz said that once the commission’s legal texts were on the table, the parliament would move quickly to expedite the legislative process.

Two of Cameron’s central demands aimed at reducing EU labour migration to Britain involve freezing in-work benefits for migrant workers, mainly from eastern Europe, and slashing their child benefits. Both changes require amendments to EU laws.

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The commission president, Jean-Claude Juncker, who has warned that the changes could affect national social security systems across the EU, insisted on Tuesday that the commission had no “plan B” prepared in the event that the summit broke down.