A Saudi princess known for her human rights advocacy work has likely been put under house arrest in Riyadh, reports said.

According to sources, Princess Basma bint Saud bin Abdulaziz al-Saud “fell off the radar”, after she attempted to leave the kingdom late last year.

The source said she was due to fly to Geneva at the end of 2018, however her flight never left Saudi soil.

Princess Basma, who is the youngest daughter of Saudi Arabia’s second ruler, King Abdul Aziz, has not been seen since then.

“She just fell off the radar; no one knew where she was. We actually feared the worst,” Princess Basma’s former attorney, Leonard Bennett told.

He added that on one occasion, the princess answered a phone call “sounding very much like a hostage”.

Another source said that the princess could not speak freely as all her communications are under surveillance.

The report added that an investigation into the princess’ alleged escape attempt had already taken place, however Saudi authorities have kept her in detention without explanation.

Princess Basma developed a reputation as an outspoken member of the Saudi ruling family, particularly through her media work.

The mother-of-five has spoken in favour of constitutional reforms in Saudi Arabia, including by expressing disappointment at Saudi Arabia’s failure to transform into a constitutional monarchy.

Princess Basma’s articles on human rights issues regularly featured in the al-Medina, al-Hayat, and al-Ahram newspapers.

In January 2018, Princess Basma called for an end to Saudi Arabia’s military intervention in Yemen during an interview.