Nick Timothy, who left his position as Mrs May’s aide last year following her election troubles, said concerns about chlorinated chicken and selling off the NHS after Britain leaves were “ridiculous”.
And he praised Brexit as a remarkable opportunity to boost the UK economy by prioritising deals with growing countries outside the bloc.
But he warned that was being put in danger by MPs trying to force the UK to stay in the customs union.
He told the Telegraph: “If we did this, we would expose ourselves to extraordinary risk.
“No country outside the EU, other than Monaco, is a member of the customs union.
As Leavers say, there is little point exiting the EU unless we are ambitious
“Not even Norway, which is in the single market, has joined it.
“This is because if they did so, EU negotiators would surrender access to their markets in return for trading rights with third countries that suit the EU, but not them.”
And he used the example of Turkey to illustrate his point, saying it was hampered by having a partial customs union with the EU.
He wrote: “Ankara cannot negotiate trade agreements with other countries unless the EU has done so first.
“EU diplomats negotiate access to Turkey’s market as well as the EU’s, but do not seek reciprocal Turkish access to the third country’s market.
Nick Timothy, former May’s aide, blasted those who spread panic about leaving the customs union
“When Ankara comes to negotiate its own deals, therefore, it has no leverage: since the EU signed agreements with Algeria, Mexico and South Africa, Turkey has failed to reach deals with those countries.”
But he agreed Remainers were correct to highlight the problem of increasing trade with other markets while keeping supply chains with Europe.
Mr Timothy said they only way around that was to avoid imposing tariffs between Britain and the bloc in the final deal.
He said: “As ever with Brexit, there is truth in the arguments of both Leavers and Remainers.
“As Leavers say, there is little point exiting the EU unless we are ambitious, go global, and seek more trade with the world’s fastest-growing economies.
Boris Johnson pleaded for unity and promised not to overturn the referendum result
“But as Remainers insist, we must not disrupt trade with our nearest and biggest export market.
“The solution therefore lies in neither extreme: we should want a comprehensive trade agreement with the EU, including regulatory cooperation and a close customs partnership, but we must leave the customs union.”
Mr Timothy’s comments come as the Government launches a PR offensive to bang the drum for Britain ahead of the next round of negotiations.
Boris Johnson kicked things off yesterday with an impassioned plea for unity and a pledge not to try and overturn the referendum result.
In comments similar to Mr Timothy’s, he warned it would be “mad” to end up with a Brexit settlement that does not allow the UK to enjoy the “economic freedoms” of leaving the European Union.
Mr Johnson said the UK needs to diverge from EU rules after leaving the bloc
Signalling the need to diverge from EU rules after leaving the bloc, Mr Johnson insisted that the UK should not remain locked into alignment with Brussels.
Mr Johnson said Britain should not be “lashed to the minute prescriptions” of a bloc comprising just 6 percent of the world’s population.
The UK has committed to leave the single market and customs union, but the EU could impose conditions to closely follow rules as part of the comprehensive deal sought by Theresa May.
Setting out his approach, Mr Johnson said: “We would be mad to go through this process of extrication from the EU and not to take advantage of the economic freedoms it will bring.”