By Stuart Cohen
BBC News, Washington
New rules go into effect on Tuesday requiring citizens of Canada, Mexico and Bermuda to produce a passport on arrival by air in the United States.
Many Canadians have been applying for passports recently
American citizens returning home from the three countries will also have to show their passports.
Until now, such travellers were only required to produce a driving licence or a birth certificate.
The new rules were passed by Congress in the wake of the 11 September 2001 attacks in the US.
Similar rules for people entering the US by sea or land will not take effect until at least next year.
Until now, all a person needed to travel between the US and Canada, Mexico or Bermuda was a driving license or birth certificate.
Even so, many times that documentation was not even looked at.
Travellers were often just asked a few questions about their nationality and travel plans.
Less than a third of Americans currently have passports, but State Department officials have reported a significant increase over the past few years in passport applications.
The tourism industry in Mexico and the Caribbean has been worried about the effect of the new rules, particularly during the traditionally busy winter travel season.
Some resorts have even offered to pay the $79 application fee for travellers who do not have passports.
To the north, Canadian passport offices have seen long queues as many Canadians have been applying for passports as the deadline for the new rules drew closer.
Just a few days ago, the US State Department issued a special exemption to the many Canadians who spend their winters in the southern US to allow them to leave the country without a valid passport when they return home.