A woman and her two sons have suffocated to death in a windowless shed to which they were banished in the latest tragedy linked to the illegal practice of chhaupadi, whereby women in Nepal are forced to sleep in “period huts”.

Police said Amba Bohara, 35, had spent four days in the cowshed with her sons Ramit, nine, and Suresh, 12, when her father-in-law discovered their bodies on Wednesday morning.

She had been confined in line with the outlawed practice of chhaupadi – when woman having their monthly periods are forced to sleep inside tiny sheds or animal shelters because they are considered impure.

Uddhav Singh Bhat, deputy police chief in western Nepal’s Bajura district, said it appeared the family had lit a fire to keep warm inside the freezing mud hut but were overcome by fumes, and flames had spread to their blanket.

“The doctors have already finished the postmortem but we are yet to get the results,” he said.

Chhaupadi was criminalised in 2005, with penalties including a 3,000-rupee (£21) fine and a three-month jail term introduced last year for those convicted of perpetuating the custom. But it remains deeply embedded in some communities, particularly in the country’s poor western regions.

The tradition, associated with Hinduism, controls what a women can eat, where she sleeps and who she can interact with during her monthly cycle. Many adherents believe that disobeying the rules invites misfortune and death.

Menstruating girls and women sit separately during a festival in Acham district, Nepal. They are not allowed to participate in festivals and rituals during their menstruation period as they are considered ‘impure’. Photograph: Narendra Shrestha/EPA

The UN has linked the practice to reports of diarrhoea, pneumonia and respiratory illnesses as well as sexual abuse or attacks by wildlife. It has also been blamed for infant and maternal deaths when mothers and babies are confined to the huts after birth.

Another woman suffocated to death in a shed in January 2018, and the previous year a teenager was bitten by a snake and died.

“Although NGOs are advocating against the blind belief and even educating the people, most women are unable to let go the tradition because of societal norms and religion,” said Judda Bahadur Rawal, the programme manager from an advocacy group in Bajura district.

He said the local government should take action against those upholding the custom. “Though the number of women practising the tradition is decreasing, there are still a number of women who practice it without being forced. The new law itself is not effective.”

Agni Shahi, NGO Federation president for Bajura, said that, despite ongoing education programmes, it had been “a real struggle to make the women let go of the belief that’s been plaguing the country”.