Popular digital assistants that reply in a woman's voice and are styled as female helpers are reinforcing sexist stereotypes, according to a United Nations report.

The vast majority of assistants such as Apple's Siri, Amazon Alexa and Microsoft's Cortana are designed to be seen as feminine, from their names to their voices and personalities, said the study.

They are programmed to be submissive and servile - including politely responding to insults - meaning they reinforce gender bias and normalise sexist harassment, said researchers from the U.N. scientific and cultural body UNESCO.

The study highlighted that Siri was previously programmed to respond to users calling her a "bitch" by saying "I'd blush if I could" as an example of the issue.

"Siri's submissiveness in the face of gender abuse – and the servility expressed by so many other digital assistants projected as young women – provides a powerful illustration of gender biases coded into technology products," it said.

Apple, Amazon and Microsoft were all not immediately available for comment.

A spokeswoman for Microsoft has previously said the company researched voice options for Cortana and found "a female voice best supports our goal of creating a digital assistant".

Voice assistants have quickly become embedded into many people's everyday lives and they now account for nearly one-fifth of all internet searches, said the report, which argued they can have a significant cultural impact.

As voice-powered technology reaches into more communities worldwide, the feminisation of digital assistants may help gender biases to take hold and spread, they added.

"The world needs to pay much closer attention to how, when and whether AI technologies are gendered and, crucially, who is gendering them," said Saniye Gulser Corat, UNESCO's director for gender equality.

The report called on companies to take action including to stop making digital assistants female by default, exploring gender neutral options and programming assistants to discourage gender-based insults and abusive language.

A team of creatives created the first gender neutral digital assistant voice earlier this year in an attempt to avoid reinforcing sexist stereotypes.

The UNESCO report was welcomed by women's groups, with Womankind spokeswoman Maria Vlahakis saying it gave "much needed attention" to gender bias in algorithms.