Campaigners in the battle over Britain's membership of the EU have traded blows over the recruitment to one of the main pro-withdrawal groups of the author of a detailed plan for life after Europe.

Prominent Eurosceptic Richard North announced on his website that the Leave.EU campaign had adopted his Flexcit model as its formal plan for the UK's exit from the 28-nation bloc, after he agreed to work with the group as a consultant during the upcoming referendum campaign.

The announcement was seized upon by pro-EU campaigners from Britain Stronger in Europe, who claimed that the model - which they branded "Still Pay, No Say" - would leave the UK in a similar position to Norway, which follows many Brussels rules and regulations but has no part in deciding them.

BSiE executive director Will Straw said the move had put Leave.EU on collision course with Ukip, because Flexcit would involve Britain continuing to pay into the EU budget and accept free movement of people.

But Leave.EU denied that Flexcit had been adopted as their formal policy, and accused BSiE of a "gross misrepresentation" of the way the plan would work. A spokesman said Dr North was "one of the country's most influential Eurosceptic thinkers" and would be helping Leave.EU develop "a credible and comprehensive exit plan" building on his 400-page Flexcit report.

Dr North's model envisages a six-stage process under which the UK would initially join the European Free Trade Association (Efta) and continue trading with remaining EU members through the European Economic Area, while repatriating the entire body of EU legislation into British law. Only over a period of years would the UK gradually implement reforms to its immigration rules and controls, trading relations and domestic governance to arrive at its final post-exit status.

Mr Straw said: "Leave.EU's new policy shows they accept EU budget contributions, would keep free movement, would keep all EU laws, but would remove the UK's influence over the most important economic regulations we would be forced to accept.

"This would give the UK all of the costs but fundamentally undermine the benefits of EU membership, removing Britain's voice at the European top table. Britain would be forced into a position where it still has to pay, but has no say.

"Leave.EU have also openly attacked and abandoned Ukip's central immigration policy, showing the utter chaos of the Leave campaigns. This is an extraordinary rejection of Nigel Farage's most defining policy position."

But the Leave.EU spokesman said: "Will Straw's gross misrepresentation of Flexcit ... betrays his woeful understanding of life outside the gilded cage of EU membership.

"Far from having to 'pay with no say', countries like Norway and Iceland participate in hundreds of EU committees, helping to shape the Single Market regulations which they apply, retaining a veto where they find them particularly objectionable. Moreover, they have full control over their agricultural policies, external trade and fishing waters, unlike EU members.

"In any case, Flexcit sees the EEA as a stepping stone rather than a final destination."

Dr North said: "BSiE clearly haven't read Flexcit, from their comments. They don't seem to have understood what's written in it anyway.

"The idea of `No say' is a complete lie, as even within the context of Efta and the EEA there are structured negotiations and consultations built into the system."

The Norwegian-style model would only be a staging post towards "our endgame of going back to Winston Churchill's original vision of an arrangement covering the whole of geographical Europe under the banner of the UN Economic Community Europe in Geneva", with nations co-operating as equals in a "European village", said Dr North.