Monday 9 November 2015 12:00 am

EU referendum: Cameron will warn he could back Brexit as new Open Europe report shows it will be difficult to win over EU counterparts

Prime Minister David Cameron will caution tomorrow that he is prepared to support Britain leaving the European Union unless other European leaders agree to his reform agenda.

Cameron is expected to spell out the UK government’s requirements for EU reform in a letter to the President of the European Council, Donald Tusk. In a separate speech, also tomorrow, Cameron will warn that if he is not satisfied with the renegotiation outcome, he will back a so-called Brexit.

Cameron has promised an in/out referendum before the end of 2017.

“If we can’t reach such an agreement, and if Britain's concerns were to be met with a deaf ear, which I do not believe will happen, then we will have to think again about whether this European Union is right for us,” Cameron will say, adding: “As I have said before – I rule nothing out.”

Cameron has come under mounting criticism from European leaders for failing to detail the nature of his desired reforms.

Meanwhile, new research out this week from a leading think tank showed just how much more the Prime Minister has to do to convince his European counterparts.

Open Europe looked at how different EU member states were likely to react to Cameron’s expected demands, finding that changing the rules on EU migrants’ access to welfare benefits in the UK was “likely to be the single-most difficult item” on the Prime Minister’s agenda. Securing stronger safeguards for non-Eurozone countries was the second most-difficult demand, followed by securing greater powers for national parliaments.

Open Europe said it would be “less controversial” for Cameron to ensure that “ever-closer union” no longer applies to all EU countries, while that the “easiest part” of the agenda would be pushing for a deeper single market.

Open Europe co-director Raoul Ruparel told City A.M. that the Prime Minister has “a lot to play for” but “quite a lot of convincing to do”.

“[Cameron] has got a lot of work to do to explain what he wants and then to get these countries on board,” Ruparel said, adding, “While other countries might be supportive, they are not necessarily going to go out and bang the drum for him.”

•Cameron will address more than 1,000 business leaders today at the annual Confederation of British Industry (CBI) conference. The CBI, which says that most its members back Britain’s EU membership, will unveil its latest growth forecasts at the conference, downgrading this year’s UK GDP outlook from 2.6 to 2.4 per cent.