The 4,000-mile-long line that separates the United States and Canada is a "sieve," say the very people entrusted with securing border between the two countries.

Ever since the Sept. 11, 2001, terrorist attacks, Canada has come under pressure from the United States to beef up border security (search).

Now pressure is coming from within Canada as well.

The union representing Canadian customs agents identified 225 roads along the U.S.-Canadian border that are currently unguarded, 50 of them in British Columbia.

The roads run right along the frontier, and in many cases there's only a ditch that separates the two countries.

Click on the video box to the right for a complete report by FOX News' Dan Springer.

The Royal Canadian Mounted Police (search) is in charge of patrolling the area, but in all the time FOX News was in Canadian border towns last week, the news crew didn't see a single Mountie or surveillance camera. The American side is watched over by the U.S. Border Patrol.

Since Sept. 11, the Department of Homeland Security has doubled the number of agents on the northern border, but it's still not enough to cover all 4,000 miles. To supplement the manpower, the United States has installed cameras every couple of miles.

Canadian customs agents complain that there has not been a similar buildup on their side.

"It's a real potential [opportunity] for terrorists to come across with the tools of the trade," said Tom Granger, a former Canadian customs agent. "Easy, come at night, drive your truck and cross the border."

Ahmed Ressam (search), the would-be "Millennium Bomber," later convicted of plotting to bomb Los Angeles International Airport on New Year's Eve, entered the U.S. by driving over the Canadian border into Port Angeles, Wash., in late 1999.

But he did so at an official border crossing, and was caught by an observant U.S. customs agent. Officials found nitroglycerin and four timing devices concealed in his car's spare-tire well.

More thoughtful terrorists could simply drive through an unofficial, unguarded border crossing.

Smuggling is also a big concern. Officials say marijuana, Ecstasy and Asian sex slaves enter the U.S. from Canada, while cocaine and guns go in the other direction.

Meanwhile, Canadian business leaders fear the consequences if a terror attack on the U.S. originates in their country.

"The closing of the border [would be] absolutely catastrophic to Canada," said Peter Holt of the Surrey, British Columbia, Chamber of Commerce. "We have 70 to 80 percent of our trade across the border to the U.S. Economically, it's catastrophic."

The ruling Liberal Party (search) in Ottawa defends the Canadian federal government's security measures.

Canada is spending more than it used to on counterterrorism measures, and has created a joint border-enforcement team with the United States.

Yet although federal officials admit manpower is lacking, they don't believe that that makes them, or the United States, more vulnerable. They say funding has increased for intelligence gathering, which is more important than putting more bodies along the border.

Roy Cullen, a Liberal member of the Canadian Parliament from Toronto and parliamentary secretary to the Minister of Public Safety and Emergency Preparedness, justifies the allocation of funds.

"The whole approach to policing and law enforcement and the fight against terrorism has changed and evolved," said Cullen. "It's now very much a predominantly intelligence-based policing."