Air France is facing a backlash after instructing female crew to wear trousers during flights to Iran and to don a “loose-fitting jacket and headscarf” before leaving the plane in Tehran.

Staff representatives have accused the airline, which will resume flights to the Iranian capital later this month, of forcing female staff to wear clothes that are an “ostentatious religious sign” that goes against French law.

Union leaders said the dress code was an attack on individual freedoms and insisted the measure had to be voluntary; Air France said the rules were not new and already applied to cabin crew during stop-overs in Saudi Arabia where hostesses were required to wear an abaya covering their body.

Air France will begin three daily fights to the Iranian capital on 17 April, eight years after they were stopped following the imposition of international sanctions against Tehran. These were lifted in January after Iran agreed to dismantle its nuclear programme.

Air France’s clothing advice was contained in an internal note to staff.

Christophe Pillet, of the SNPNC union and a member of the Air France staff committee, said the instructions had sparked widespread concern. “Every day we have calls from worried female cabin crew who say they do not want to wear the headscarf,” Pillet told AFP.

He said airline management had raised the possibility of penalties against staff who refused to follow the dress code.

Françoise Redolfi, another union leader, told RFI radio, “They are forcing us to wear an ostentatious religious symbol. We have to let the girls choose what they want to wear. Those that don’t want to must be able to say they don’t want to work on those flights.”

She added: “Many female members of flight crews have told us that it’s out of the question they be obliged to wear headscarves. It’s not professional and they see it as an insult to their dignity.”

Since the 1979 Islamic revolution, women in Iran have been required to cover their heads. In France, headscarves are banned in schools and colleges, and full veils are outlawed in public places.

In a statement, Air France said: “Iranian law requires the wearing of a veil covering the hair in public places for all women present on its territory. This obligation is not required during the flight and is respected by all international airlines serving the Iranian Republic.”

The row comes days after the French women’s rights minister, Laurence Rossignol, sparked controversy after comparing women who choose to wear the veil – including the burqa and niquab – to “American negroes who supported slavery”. Rossignol had been asked about fashion companies producing clothing for Muslim women, including Marks & Spencer, which has included ‘burkinis’ – all-covering swimming costumes – in its 2016 summer collection.

In 2010, Laurence Ferrari, one of France’s most famous television news presenters, caused outrage after wearing a headscarf to interview the Iranian president, Mahmoud Ahmadinejad.

On Sunday, leading French feminist philosopher Elisabeth Badinter called for women to boycott companies including Dolce & Gabbana, H&M and Marks & Spencer for producing “Islamic fashion”.