Air Canada must allow passengers to take along their pets on its domestic and international flights, the Canadian Air Transportation Agency ruled Friday.

The airline changed its policy last summer so that passengers could no longer transport cats, dogs and other small animals on planes as checked baggage on the grounds that more space was needed to accommodate luggage. Instead, passengers were told their pets would have to travel separately aboard cargo planes.

In its decision, the agency called Air Canada's policy "unreasonable" because it favours the air carrier while exposing pets to harm.

The airline has until May 5 to comply with the ruling and allow animals and their crates, with a combined weight of less than 31 kilograms, to be transported on all flights.

The investigation followed a complaint from a passenger, Peter T. Griffiths, who said the policy would cause considerable inconvenience to people travelling with their pets and increase their costs.

"The fee I would be charged would more than double from the current $105 to $220 each way for my six-kilogram dog," he wrote. "It is very clear the sole purpose of this decision is the financial gain in the very lucrative air cargo market."