A British exit from the European Union would pose "unpredictable political and economic risks", the country's leading Eurosceptic thinktank argues in a report that will have a significant impact on the debate in the Tory party (pdf).

As a growing number of Conservative MPs join the anti-EU Better Off Out campaign, the Eurosceptic Open Europe thinktank hits back by arguing that membership remains "the most beneficial arrangement" for Britain.

The report, seen by the Guardian before publication, says: "From purely a trade perspective, EU membership remains the best option for the UK. All the alternatives come with major drawbacks and would all … require negotiation with and the agreement of the other member states, which would come with unpredictable political and economic risks."

The intervention by Open Europe comes after George Osborne highlighted how Britain is inextricably linked to the EU when he warned that the crisis in the eurozone is jeopardising economic recovery. "Our recovery, already facing powerful headwinds from high oil prices and the debt burden left behind by the boom years - is being killed off by the crisis on our doorstep," the chancellor wrote in the Sunday Telegraph.

The Open Europe report, by Stephen Booth and Christopher Howarth, endorses Osborne's view as it warns that "economic stagnation" means Britain must do better in exporting to growing markets outside Europe. But the report throws its weight behind continued British membership of the EU as it rejects a series of alternatives advocated by hardline Eurosceptics, who claim Britain could preserve its exports to the EU – 48% of UK goods and services exported go to EU countries – even if it left.

These alternatives include:

• The "nearly but not quite an EU member" model embodied by Norway which has access to the EU single market as a member of the European Economic Area but little power to shape EU rules.

The report says: "Norway as an EEA member has to abide by EU regulations with regards to the single market such as product regulations as well as EU social and employment laws. This is a significant body of legislation over which Norway has no voting rights, although it does have some influence."

• The so called "Swiss option" of a free trade deal with the EU. "The UK's access to the single market would be dependent on the deal it could negotiate with the EU – the Swiss deal currently excludes the vast majority of services, including financial services," says the report.

• A "stripped-back customs union with the EU" – known as the "Turkish option" – in which Britain would have access to the single market but would be bound by external trade deals agreed by the EU. This would lead to an "asymmetric and dependent relationship".

• The "clean break" in which Britain would leave the EU and rely on its membership of the World Trade Organisation to negotiate trade deals. The report warns: "There would be a major price to pay for this. In the absence of preferential trade agreements with either the EU or other countries, exporters in the UK would suddenly be faced with new tariffs."

Open Europe does, however, reject the status quo as it argues for a major reform of Britain's relationship with the EU. It says Britain should remain a full member of the single market in goods and services but should take a "pick and mix" approach in other areas by, for example, leaving the common agricultural policy and the common fisheries policy.

Mats Persson, the director of Open Europe, said: "Given the growing public hostility to the EU and events in the eurozone, the status quo isn't an option. Therefore, it is in the UK's interests to stay in the EU but renegotiate a new model for membership founded on a continued commitment to EU-wide trade but substantially less EU involvement in other areas."

The Open Europe report will strengthen the hand of the moderately Eurosceptic Fresh Start group of Conservative MPs who want to remain in the EU but who want to renegotiate the terms of British membership. They are currently battling Tory hardliners who want to use the eurozone crisis as a reason to hold a referendum on whether Britain should remain a member of the EU.

Many Tory MPs have seized on a report by Lord Owen which calls for a referendum to allow Britain to join a looser European community outside the formal structures of the EU. Douglas Carswell, a leading Tory member of the Better Off Out group, is attempting to use a Commons private member's bill to repeal the original legislation which allowed Britain to join the EEC in 1973.

Osborne has highlighted nerves among senior Tories by pointing out the government has introduced a "referendum lock" to be triggered if there were a transfer of UK sovereignty to Brussels – different circumstances from those envisaged by Owen.

George Eustice, a leading member of the Fresh Start group, suggests in the Guardian article today to give a flavour of the tactics the government might adopt on a referendum. He suggests that a referendum could be held to endorse a renegotiation of British membership suggested in the Open Europe report.

Eustice writes: "While the time for a referendum may come, it must offer the public the option most want which is to stay in the EU but under new terms."