(Adds responses from Avianca, pilots union)
BOGOTA, Oct 6 (Reuters) - A Colombian tribunal on Friday declared illegal a strike by the majority of Avianca Holdings SA's local pilots, a decision that may hasten the end of nearly three weeks of travel headaches for thousands of passengers.
Pilots from the Colombian Association of Civil Aviators, or ACDAC, began the walkout on Sept. 20 and are demanding salary and benefits changes they say will put them on par with the airline's pilots in other countries. More than 700 of the company's 1,300 Colombian pilots voted to strike.
The airline has been forced to cancel more than half of its flights on its domestic and international routes, affecting more than 300,000 passengers.
The Superior Tribunal of Bogota ruled that the strike is illegal because it interferes with essential public transport. The decision opens the door for pilots to be fired for taking part in the strike, which may push the union to call it off.
ACDAC told journalists it would appeal the ruling, while Avianca said in a statement it hoped the pilots would return to work as soon as possible.
The airline has said the strike is illegal and called the pilots' demands unreasonable. Pilots want reduced working hours and for Avianca to pay 70 percent of their monthly taxes, among other things.
In an effort to mitigate the effect of the cancellations, the civil aviation authority this week allowed the airline to bring in foreign pilots to fill in on some routes.
Avianca, a member of the Star Alliance and one of the leading airlines in Latin America, carried 29.5 million passengers in 2016. It has more than 21,000 employees and serves 105 destinations in 28 countries in the Americas and Europe. (Reporting by Julia Symmes Cobb; Editing by Andrew Hay and Diane Craft)