Boris Johnson has said the so-called "Project Fear" is being used to keep the UK in the Customs Union and to “make a nonsense” out of Brexit.
The former foreign secretary spoke at the Pendulum Business and Self-Empowerment Summit in Dublin on Thursday and said “there was still time to resolve the Irish border issue”.
Mr Johnson’s comments come as the Prime Minister met with Japanese Prime Minister Shinzo Abe, who has formally backed Mrs May’s Brexit deal.
He said his country was in “total support” of the Withdrawal Agreement ahead of next week’s vote in the Commons.
Mr Johnson said the UK and the Republic of Ireland "should be bold and brave together" over the border issue.
He said: "Whatever you may think about what people voted for, at present the arguments, the whole Project Fear stuff is being used to keep the UK in the Customs Union, in the Single Market and therefore really to make a nonsense of leaving the EU.
"We are told we can't do a deal with our friends over the Channel, if we can't do a deal with our friends across the Channel then there is a real risk of a hard border in Northern Ireland and a risk also that the UK will run out of drinking water, out of two crucial ingredients for Mars bars.
"Nobody, not in Dublin, not in London, not in Brussels, no one wants a hard border in Northern Ireland.
"No one is going to accept it, no one is going to implement it and we shouldn't, simply because of these apprehensions, abandon the attempt to find the technical solutions that are readily available - according to the HMRC, according to the Irish authorities - without even trying."
The former Cabinet minister said that no one will accept a hard border between the Republic and Northern Ireland.
He said: "No one is going to accept it, no one is going to implement it and we shouldn't, simply because of these apprehensions, abandon the attempt to find the technical solutions that are readily available - according to the HMRC, according to the Irish authorities - without even trying.
"If there is, as we are told, a tiny risk of a shortage of whey for Mars bars, I say let's sort it out. Where there is a will there is a way.
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"I would have thought, faced with what is a logistical and bureaucratic and technical problem, our two great countries should be bold and brave together and self-confident about our shared future because we are so alike in so many important ways.
"No matter what nonsense the prophets of doom may talk about the risks of new sanitary checks on livestock or hold up on the border in the event of Brexit, I know that together we can organise it in the interests of our business.
"We have time to do it, we have time to get this right.
"I think if we can get it right together over the next few months, as I am absolutely sure we can, that the pendulum will swing again as it always does, away from this temporary gloom towards a new spirit of Anglo/Irish optimism and self-confidence, and when it does it will be the entrepreneurs, yourselves included, that we will have to thank."