Supporters of a British exit from the EU are peddling an “impossible fantasy”, the chairman of the main pro-EU campaign Stuart Rose has warned.

As David Cameron intensifies his EU negotiations with a flying visit to Budapest, Lord Rose accuses the heads of the two groups campaigning to quit the EU of making a series of claims about a new settlement with the EU which may never be achieved.

“This is nothing short of an impossible fantasy,” the chairman of the Britain Stronger in Europe campaign tells his counterparts in Vote Leave and Leave.EU, John Mills and Arron Banks.

Rose stepped up the pressure on his opponents as the prime minister wooed Germany during a visit to the CSU conference in Bavaria by saying that he shares their support for the EU’s historic commitment to the principle of free movement of people.

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Cameron will fly later on Thursday to Budapest for talks with Viktor Orbán, the rightwing Hungarian prime minister, who has said he wants to help the UK in its EU renegotiations which may come to a head at a Brussels summit next month.

Amid fears among some pro-EU campaigners that the Vote Leave group has made the running in the EU debate in recent months, Rose will take aim at what the anti-EU campaigners admit is their greatest weakness. This is the fact that they cannot definitively say what settlement the UK would negotiate with Brussels outside the EU because Britain does not have a decisive say in the process.

Under EU rules Britain would trigger article 50 of the Lisbon Treaty if a vote to leave is recorded in the referendum. At this point the UK would lose its seat in the European council, whose remaining 27 members would appoint two EU prime ministers to negotiate a new relationship between the UK and Brussels outside the EU. These negotiations would take place over two years and would have to be approved by the European parliament.

In his letters to his opponents Rose says that their claims about a new settlement are baseless. He wrote: “The public deserve to be told how you believe Britain can prosper outside Europe. You claim that Britain can have all the benefits with none of the costs. You promise to simultaneously end free movement; end all contributions to the EU budget; opt out of economic rules and regulations; whilst still retaining full access to the single market.

“This is nothing short of an impossible fantasy. If you can demonstrate otherwise, I’m more than happy to have that debated. But it’s simply not acceptable for your campaign to continue to duck questions about what ‘out’ looks like. You must produce hard evidence to back up your assertions and be realistic about the economic consequences.”

Facebook Twitter Pinterest Stuart Rose, chairman of the Britain in Europe campaign. Photograph: Dan Kitwood/Getty Images

The pro-EU campaign has produced a video mashup to highlight the contradictory claims made by anti-EU campaigners. Nigel Farage, the Ukip leader, is shown saying that Britain could follow the “Norway option” which has allowed Oslo to enjoy access to the single market without joining the EU. Douglas Carswell, the Ukip MP, is shown declaring that nobody on his side supports that option.

Farage is also shown saying that he did not want to be part of the single market – the heart of the “Norway option”. Dan Hannan, the anti-EU Tory MEP, is shown saying that “absolutely nobody” is talking about threatening the UK’s place in the single market.

Pro-EU campaigners believe the confusion among anti-EU campaigners about Norway is a good illustration of central flaws in their argument. They talk about ending Britain’s financial contributions to the EU and of being exempt from the EU’s rules on free movement. But Norway is the tenth largest contributor to the EU budget and is bound by the rules of the single market without any vote. Oslo is a member of the European Economic Area, whose members are bound by free movement.

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James McGrory, campaign spokesman for Stronger In, told the Guardian: “The video is a bit of fun but it makes a serious point – the leave campaigns won’t be straight with people about what they are actually proposing. This will be the focus of much of our work over the next few months – pinning them down to a specific proposal, whether it’s their fantasy free-trade model or Britain being like Norway or Switzerland.

“They’ve got to come clean on this. They are either being deluded or deceitful in pretending that it’s all gain and no pain if Britain were to leave the EU. They can’t be allowed to duck and weave forever. It’s about time they told us so we can have a proper debate on the facts, not in fantasy-land.”

In his letter Rose challenges his counterparts to answer five tests. He asks them how they will:

Gain the support of EU member states



Retain full access to the single market

Maintain inward investment in the UK

Preserve UK trade

Protect British jobs

Rose writes: “If you cannot meet these five tests, it will be clear that you do not have a credible or achievable alternative to EU membership. I do not know whether you cannot or will not be clear about what ‘out’ looks like but the British people deserve an honest assessment of the benefits and costs of leaving the EU. Come clean on your proposals and then let’s have this important debate.”

Arron Banks, co chair of the Leave.EU group, described the claims by Rose as overblown. Banks told the Guardian: “Britain is the fifth largest economy in the world. The EU exports £60bn to £70bn more in goods to the UK than we export to the EU. We are the largest export market for the EU. We would be able to negotiate a fantastic deal. This is just scaremongering.”

Robert Oxley, a spokesman for Vote Leave said: “Countries across the world trade with the EU without being subject to the supremacy of EU law or paying Brussels £350m per week – 60 times what we spend on the NHS cancer fund. We will negotiate a UK-EU deal based on free trade and friendly cooperation that will be better for us and Europe. The BSE campaign has reverted to baseless scaremongering, ludicrously they are even trying to attribute our trade with the USA as a supposed benefit of the EU.”