Co-Chairman of Leave Means Leave, John Longworth, argued pro-Brussels politicians in the establishment could attempt to backtrack on Brexit.
Speaking to the BBC, the former Director-General of the British Chambers of Commerce, added opting to become a member of the European Economic Area (EEA) would mean undermining the EU referendum result.
Mr Longworth said: “When the campaign took place to leave the European Union, all the people who supported Leave made it clear what Leave means.
“That it means taking control of our own borders, having Parliamentary Sovereignty, having our courts rule over the UK and not the European Court of Justice and having the freedom to create a better economy for ourselves.
“And if we actually take the Norway option many of those things will be undermined. Now both the major parties, Labour and the Conservative Party, have in their manifestos that they want to Brexit.
“The Labour Party made it just as clear as the Conservative Party that they want to leave the single market which would then enable us to do many of the things that I have outlined.
“What we have got to make sure of is that the people within the establishment of government don’t undermine [that].”
Mr Longworth added: “Remainers want Brexit in name only but in actual fact, of course, we’ll be in for all intents and purposes.”
On Tuesday, Tim Farron called on Theresa May to give the Liberal Democrats a seat at the Brexit negotiating table as he argued the party was prepared to compromise on their demands.
The pro-EU politician said it was time Westminster stopped “stop fighting like cats in a sack” and cooperated in the national interest.
Speaking to Sky News, Mr Farron said: “I actually think for the good of the British people, for the good of our country, it needs to last as long as possible.
“The Prime Minister needs to show humility, she made a hubristic, self-interested choice to call and election seven-eight weeks ago.
“It’s damaged her, it’s damaged the country, it’s divided the country, but we are where we are and we have to work to gather and make this work.
“Fundamentally, most importantly, on the issue of Brexit. We’ll all have different views but this is a moment where the Prime Minister should, in my view, set up a joint-Cabinet committee and she should make sure it’s an all-party one.
“Yes, she should Chair it, but there should be Labour, Liberal Democrats, and Scottish Nationalists on that committee also.”
Mr Farron insisted there was interest in cross-party cooperation, as he claimed Mrs May did not have a mandate to go ahead with the Tories’ manifesto.
He said: “And we should own the negotiation together, stop fighting like cats in a sack, and start behaving like grown-ups working in the national interest.”
June 14th, 2017: Express