On a Tuesday morning in June, Elham Asghari stepped into the tidal waters of the chilly Caspian sea in northern Iran to swim 20km in full Islamic dress. But her record-breaking nine-hour feat has not been recognised by national authorities because she is a woman.

"Although I [stuck to] the full Islamic dress code and had swimming officials present at all times, [the authorities] said no matter how Islamic my swimming gear, it was unacceptable," she told the Guardian. "They said the feminine features of my body were showing as I came out of water."

Swimming in open waters had been Asghari's childhood dream. To achieve her goal, she looked for training programmes on the internet and came up with the idea of designing a special swimsuit – a full hijab, covering her body from head to toe. It adds some 6kg to her weight in water and, she says, it is painful to wear.

Last month, although she broke her previous national swimming record, Iranian authorities refused to recognise her achievement.

In frustration Asghari posted a video of herself online with the help of her manager, Farvartish Rezvaniyeh, who decided to help publicise her plight when he heard about it on Facebook. "I could not believe this injustice was happening to a record-breaking champion. I contacted her … and we made the video," he said.

The footage, which includes the 32-year-old swimming in her Islamic swimsuit and appealing to her fellow Iranians for support, quickly caught the attention of thousands of people who shared the video clip on social networks. Tributes poured in as more people became aware of her cause.

In the video, posted on YouTube and viewed by at least 120,000 people, Asghari promises not to give in to pressure. "No swimmer will ever accept to swim with such swimsuits; swimming with these swimsuits always hurts my body," she says in a voiceover as she is seen swimming in a pool.

"I swam 20km in [the northern city of] Nowshahr, they lowered it to 15km. I protested and they accepted 18km. Yet now, they do not register the record.

"My 20km record has been held hostage in the hands of people who cannot even swim a distance of 20 metres. I have passed tough days and nights. This incident is unbelievable for me. I will not give in to pressure. Swimming is not exclusively for men – we ladies do well too."

Women in Iran can use public swimming pools at gender-segregated times, or women-only sections, but sports officials are reluctant to allow them into open waters. "They fear that if they recognise my record then they would unwittingly approve my swimming gear and that would eventually give women swimmers access to open waters," Asghari said.

She started swimming aged five, she said. "Sometimes I feel I am an amphibian, capable of living both on land and in water. In a 24-hour [period], I spend as much time on land that I spent in water. My father was a veteran wrestler … it was him who encouraged me to register my records."

In a previous open-water race near the southern island of Kish, Asghari said police boats tried to stop her in a dramatic sequence of events that led to her leg and hip being sliced by the vessel's propellers.

After battling various gender-related obstacles during President Mahmoud Ahmadinejad's eight years in office, Asghari has pinned her hopes for change on the newly elected leader, Hassan Rouhani, who will be sworn in in August. "I hope that in President Rouhani's government, these people [hindering my career] will have no place. I will definitely follow up the case about my swimming record [when he takes office]."

Iran prevents female swimmers from participating in overseas competitions. The Women's Islamic Games in Tehran is one of the few international events where domestic swimmers are permitted to take part.