Israel's El Al Airlines is facing criticism for a new policy that requires female flight attendants to wear high heels until all passengers have boarded.

Under a previous policy, female El Al flight attendants were required to wear high heels at the airport, but could change into flats on the plane before passenger boarding.

The airline has defended its directive to attendants to wear "presentable shoes."

“It should be noted that this practice is accepted in the airline industry worldwide,” an airline spokeswoman said. However, the workers union of El Al instructed its female flight attendants Tuesday to ignore the new set of rules.

“I am not convinced that high heels are an absolute condition for women’s presentability, and certainly not for a female flight attendant who is required as part of her job to be on her feet for extended periods,” Galia Wallach, CEO of women's group Na’amat, wrote in a letter to El Al CEO David Maimon, according to The Jerusalem Post.

The union said the policy would impact the flight attendants' safety and health, calling El Al's stance "aggressive."

El Al is not the first airline to receive criticism for policies requiring female flight attendants to wear high heels. In 2008, UK watchdog Trades Union Council criticized British airlines with similar policies "blatantly sexist."

Several aviation professionals have also criticized the decision as unethical, unhealthy and possibly counter to safety, according to Haaretz.

"The company revises its service protocols regularly and it was decided in that framework to require female flight attendants to wear formal shoes while receiving the passengers to the flight," El Al's Vice President Customer Services Yehudit Grisaru said in a statement. "Immediately after the seating phase and throughout the flight itself, they wear work shoes."

@haaretzcom Male and female flight attendants must wear high heels I presume :) — suzanne fitzpatrick (@suzannefitzp) June 16, 2015

Hundreds of ELAL Israeli flight attendants signed a petition against the company for forcing them to work in high-heels. — Rebecca Griffin (@dorothyofisrael) June 14, 2015

I'm in favor of the "No High Heels" protest, over El Al Airlines requiring female flight attendants to wear high heels. — Sherri (@SherriPizza) June 15, 2015

A study released earlier this month in the U.S. found that the number of high-heel related injuries has spiked in recent years.

Female Israeli lawmakers and a women's group have called on El Al to dump the policy.

Additional reporting by the Associated Press