Conservative Brexiters are attempting to reignite opposition to Theresa May’s withdrawal agreement by claiming it is based on “defeatist” fears and scare stories about leaving the customs union.

A pamphlet launched by the European Research Group and Global Britain says May’s negotiating strategy has been based on a misunderstanding about how international trade works.

Amid fears that a rebellion against the prime minister and her deal has stalled, the paper outlines what two groups claim are 17 myths about leaving the customs union.

These includes challenging the idea that a deal based on World Trade Organization rules would require checks at the borders.

Lord Lilley, one of the authors of the pamphlet, said: “The proposed EU agreement is the result of defeatist negotiating stemming from a complete lack of understanding about how international trade actually works. We should reject Theresa May’s constitutional monstrosity of a proposal.”

Speaking on BBC Radio 4’s Today programme Lilley criticised the prime minister for rejecting an offer by the president of European council, Donald Tusk, for a Canada-style free trade agreement.

Lilley said: “The reason they give for turning it down was partly because they were afraid it might not apply to Northern Ireland … but also because they said a free trade area introduces friction in trade because companies have to lodge customs declarations and declare the origins of their goods.

“I’ve published a paper today saying that all these fears ... are at best exaggerated and at worse completely imaginary.”

When Lilley was challenged about claims of frictionless trading under WTO rules, by the BBC’s reality check correspondent Chris Morris, he accused the corporation of pro-remain bias.

Morris said: “The vast majority of trade experts and freight companies would disagree with that fairly robustly. Being in the single market and the customs union does provide trade which is entirely frictionless in a way that the government wants and in a way that it needs in order to keep the Irish border open, that a free trade agreement would not entirely bring in.”

Lilley replied: “You are supposed to be a reality check not an arguer for a remain.” Later he added: “I would love you to see the same degree of rigour applied to those putting forward the arguments for the remain side.”

Lilley refused to discuss a meeting he and other Brexiters had with Theresa May on Monday.