An American immigration lawyer is predicting that more Canadians could be denied entry into the U.S. after recreational cannabis becomes legal, and is urging the federal government to take steps to inform Canadians about the implications of legal cannabis at the border.

“I see a wall on the northern border because of marijuana,” Len Saunders, an attorney based in the state of Washington near British Columbia, told a federal senate committee hearing on Monday. The committee on national security and defence was discussing Bill C-45, the recreational cannabis legalization law that’s set to come into effect this summer.

Under the legislation, adults in Canada will be allowed to purchase cannabis for recreational use, however the substance is federally illegal in the U.S. While Canada’s Public Safety Minister Ralph Goodale has discouraged Canadians from lying to American border guards if questioned about their marijuana use, Saunders said there can be grave consequences for being truthful — including being barred entry to the U.S. for life, something that he’s seeing his clients face on a regular basis.

He said he used to see a couple cases per month involving Canadians being denied entry after admitting to using cannabis, but that has now risen to one or two cases per week, and includes a high proportion of young people.

Saunders cited studies that estimate around 40 percent of Canadians will buy recreational cannabis once it’s legalized. “That means that 40 percent of Canadians technically could be deemed inadmissible to the U.S.,” Saunders told the committee.

“It’s a booming business [for me] and it’s unfortunate because I look at myself and I think I should be helping people immigrate to the U.S.” instead of seeing them spend thousands on legal feels because they were interrogated at a point of entry, he said.

He added that not every U.S. border agent asks travellers whether someone has consumed cannabis in their lifetime — this type of question is entirely discretionary — and Canadians are not required to answer it.

“The worst case is a simple denied entry,” said Saunders. “What they do is they interrogate you … I see this intimidation and people eventually break down [and admit it].”

Saunders pointed to Canadian Olympic snowboarder Ross Rebagliati who admitted to using cannabis decades ago on the Jay Leno show and now requires a special waiver to enter the U.S. Those can cost hundreds of dollars, he said, and he thinks more Canadians will apply for them after legalization.

He suggested that even Prime Minister Justin Trudeau may require the waiver once he no longer has a diplomatic passport due to his previous admissions that he’s smoked cannabis.

For Jenna Valeriani, an advisor at the Canadian Students for Sensible Drug Policy, Saunders’ comments provide some clarity on the possible consequences for Canadians crossing the border after legalization, including for those who work in the legal industry. The committee heard of a Canadian cannabis consultant who was denied an American investors visa and barred from entering the country because of his line of work.

“This blanket denial of entry is going to be really problematic for Canadians, and to foster folks to not be honest when they’re crossing,” said Valeriani in a phone interview. “There are larger implications that need clarity and it needs to come from the Canadian government and they need to engage the U.S. on a conversation about what this means for Canadians.”