Hundreds of people are suddenly discovering that they are not Canadians as new laws requiring travellers to have a passport to fly to the U.S. go into effect Tuesday, CBC's investigative unit has learned.

Many applying for a Canadian passport have been informed their chance to remain a citizen expired years ago because of an obscure provision in the Citizenship Act, a little-known law that applied between 1947 and 1977.

The law states that if you lived outside Canada on your 24th birthday and failed to sign the right form, you automatically lost your citizenship.

Barbara Porteous applied for a passport last year and was told in a letter from Citizenship and Immigration that she would have to apply to become a landed immigrant after spending most of her 70 years in Canada.

"These documents confirm you were a Canadian citizen, but you ceased being a Canadian citizen on June 14, 1960, the day following your 24th birthday," the letter read.

A Canadian born in the U.S. to a Canadian father, Porteous has lived in Osoyoos, B.C., for the last 46 years and even worked as a returning officer for Elections Canada.

"I cried for a couple of hours," Porteous told CBC News. "I mean, the hollowness you get inside when you find out that everything you live for is gone."

Porteous is part of a group known as the Lost Canadians. According to Canadian census data, there are an estimated 10,000 to 20,000 people in Canada who could find out they've lost their citizenship if they apply for a passport.

Porteous said her life could be ruined by a technicality she was never told about.

"Well, this is my fear, because I've been getting my pension for five years. Do they want it back with interest? Does my medical go out the window, too? I'm 70 years old."

'They took my birthright away'

Don Chapman of Gibsons, B.C., recently joined a line of people shuffling through security at a federal building in Ottawa to lobby politicians on behalf of people who have lost their Canadian citizenship.

Chapman was born in Canada to Canadian parents, but 34 years ago, he was told he is not Canadian.

"I was born in Canada," he told CBC News. "My father, when I was a child, took out American citizenship. So, they took my birthright away."

Citizenship and Immigration Minister Diane Finley acknowledged the provisions are unfair and said the government would shift its policy to fast-track the process of becoming a citizen for these people.

Prior to this week, Canadians without status would have to apply to become landed immigrants — a process that takes three years or more.

Now, they will be able to apply for a grant of citizenship in just eight months.

"We're trying to right the wrongs of the past and do the reasonable thing, the right thing, for what are essentially Canadians in all but name," Finley told CBC News in an exclusive interview.

But critics say that still leaves people like Porteous in limbo for too long. 

Liberal MP Andrew Telegdi, vice-chair of the citizenship and immigration committee, called for Parliament to pass a new law for the Canadians who should never have lost their citizenship in the first place.

"I mean, it just defies logic," Telegdi told CBC News. "The system doesn't make any sense, so it's critical that we have a citizenship act that is in compliance with the Charter of Rights and Freedoms and the generosity of what Canadians believe."