Blow to Cameron's EU push as Brussels parliament chief admits "no guarantee" MEPs won't veto deal

The developments came as controversy surrounded remarks by Prince William, who, during a speech at the Foreign Office, appeared to make a tacit call for the UK to stay in the EU. It stirred memories of the Queen appearing to intervene to the No camp’s benefit in the Scottish independence campaign when she urged voters to “think carefully” about how they cast their ballots.

The future king said the nation's ability to "unite in common action" with other countries was essential in the current "turbulent world," and was the "bedrock of our security and prosperity".

But Kensington Palace was quick to insist that the Duke of Cambridge’s words were “not about Europe”.

As the Prime Minister continued his frantic European charm offensive with just 24 hours to go before he travels to Brussels for the crunch two-day European Council, he met Mr Schulz and members of the main political groups in the European Parliament.

While his reform programme will be agreed – or not – by the heads of government, the details, particularly on the proposed welfare curbs for migrants, will have to be ratified by MEPs.

No 10 is adamant that the deal would be “irreversible and legally binding”, pointing out how the heads of government were leaders of political parties and, while there was a so-called “co-decision” with the European Parliament, the expectation was that the parties would agree to the deal signed by their national leaders.

But Mr Schulz pointed out: “To be quite clear: no government can go to a parliament and say: 'This is our proposal, can you give a guarantee about the result?' This is, in a democracy, not possible.

"Therefore, my answer is the European Parliament will do the utmost to support compromise and a fair deal but I can't pre-empt the result in the European Parliament.”

Hungarian MEP Gyorgy Schopflin agreed the European Parliament could “wreak havoc”.

He explained: “That's a distinct possibility. That depends on the shape and concept of the deal; we won't know this until the early hours of Friday morning.”

MEP Nigel Farage, leader of the anti-EU Ukip, noted: "There are many groups here, who are spoiling for a fight. The real truth is that this deal is not worth the paper it's written on. It is subject to European Parliamentary approval and ultimately judgements of the European Court of Justice."

Other developments included:

*Tomas Prouza, the Czech minister for Europe, said the proposed changes to child benefit would only apply to newcomers rather than existing claimants and that other EU countries should not be able to follow the UK's lead;

*Mr Tusk confirmed the proposed deal meant EU citizens currently working in the UK would not be affected by the proposed curbs on in-work benefits, meaning it would only apply to new migrants;

*Jean-Claude Juncker, the European Commission President, said there was no Plan B for Brexit, insisting: “Britain will stay in the European Union as a constructive and active member of the Union;"

Downing Street said Mr Cameron and Mr Juncker had agreed that the talks had "progressed well" since the publication of the draft deal but "there are still details to be nailed down" to secure an agreement later this week.

Meantime, the Leave camp was embroiled in more controversy.

The Go Movement had announced that Eurosceptics from the Grassroots Out campaign, Ukip and Leave.EU would come together under its umbrella and apply for designation as the official Leave campaign.

It claimed its application for Electoral Commission designation had the support of politicians from the Conservatives, Labour, Ukip and the DUP, each with their own separate Go group.

The DUP's was to be headed up by MP Sammy Wilson, the movement claimed, but a party spokesman insisted there was never any such agreement. “It's utter codswallop,” he said.