(Reuters) -Flight attendants at United Airlines have voted in favor of a strike authorization, the Association of Flight Attendants-CWA said on Wednesday.

Over 90% of the flight attendants participated, with 99.99% of the votes in favor of a strike authorization, the union said.

It is the first time since the 2005 bankruptcy negotiations that flight attendants at United voted on strike authorization, it added.

Over 28,000 flight attendants of the Chicago-based carrier affiliated with the AFA have been negotiating for a double-digit base pay increase and higher pay for time at work - including on-ground duties, retroactive pay, schedule flexibility and work rule improvements.

"As Labor Day travel begins, United management is reminded what's at stake if we don't get this done," said Ken Diaz, president of the United chapter of the AFA.

The situation unfolds amid new contract negotiations in the U.S. and Canada, where cabin crews at carriers are also seeking compensation for on-ground duties such as boarding passengers and waiting between flights.

Last year, United's flight attendants filed for federal mediation through the National Mediation Board (NMB).

The union added that it can now request a release from the NMB leading to a 30-day "cooling off" period and strike deadline.

Even though the strike is authorized, the flight attendants cannot walk off the job until they're granted permission from the board after it determines that both parties are at a standstill and further negotiations would not be productive.