Border agents will be able to share information about U.S. and Canadian citizens more freely as the third phase of a 2011 border agreement meant to make it easier for trade and travel across the Canada-U.S. border comes into effect.

It was announced on Thursday that U.S. Customs and Border Protection and the Canada Border Services Agency will exchange biographic data, travel documents, and other information related to border crossings of U.S. and Canadian citizens.

Since 2012, the two countries have shared similar data about legal permanent residents and third country nationals.

The changes are part of the "Beyond the Border" security agreement started in 2011 by former prime minister Stephen Harper and former U.S. president Barack Obama.

A passenger holds a Canadian passport before boarding a flight in Ottawa. The new regulations mean data collected at the border about U.S. and Canadian citizens will be shared between the two countries. (Tom Hanson/Canadian Press)

A news release from the U.S. Department of Homeland Security says the data sharing will help both governments determine how long a person has been in or out of the country, and identify those who have overstayed their period of admission.

Acting U.S. Department of Homeland Security Secretary Kevin K. McAleenan said in a statement that he is proud of the co-operation between the two countries.

"Ultimately, our commitment to sharing information on travellers moving across our borders helps improve public safety, detect dangerous actors and those who violated their visas, and enforce our rule of law."

Controversial move

The information-sharing agreement has had its share of detractors.

During the Senate debate around Bill C-21 — the act that implemented the aforementioned border security measures — concerns were raised about privacy.

Last year, the International Civil Liberties Monitoring Group told CBC News that Bill C-21 is "not the most menacing or far-reaching piece of legislation" — but said it feared the push to collect more and more information about Canadians is part of a larger trend toward national security overreach.

Len Saunders, a U.S. immigration attorney based in Blaine, Washington, says while it's too late to change the law, Canadians should be wary about how their information is being shared.

"This is basically a slippery slope here," Saunders said. "I don't think that's any of the Americans' business ... And where does it stop? What other kind of information are they in the future going to allow the Americans to access?"

In the release, Ralph Goodale, Canada's Minister of Public Safety and Emergency Preparedness, said the legislation will address privacy concerns.

"The Government of Canada is determined to keep our border secure while protecting individual rights and freedoms, and has built privacy protections into the core of the Entry/Exit initiative."