New rules about adult content on blogging site Tumblr, which have upset many of its users, have come into force.

The site no longer allows such material but said existing posts that fell foul of its new policy would be hidden, not deleted.

It also tried to reassure users that Tumblr would remain a place for free expression.

However, in response to concerns about the accuracy of its software, Tumblr acknowledged it currently often wrongly blocked content because of poorly performing algorithms.

"Having a post mistakenly flagged as adult totally sucks,” the company said in a blog post on Monday.

“We understand and agree that there have been too many wrongfully flagged posts since we announced the policy change.”

It said it would encourage users to report incorrectly flagged content in order to help train its system.

Unhappy users accused the company of taking too blunt an approach to adult material instead of investing in more targeted ways of cracking down on the posting of illegal material.

"Tumblr is being lazy by banning all adult content," one user wrote. "They're punishing those of us who depict adults being adults."

Non-mainstream
The decision to disallow adult content came after Tumblr was removed from the Apple App Store last month. Last week, once Tumblr had announced its new policy, it was reinstated.

The new guidelines prohibit any depiction of human genitalia, female nipples or sex acts, including illustrations. There will be exceptions for classical artistic works and political protests that involve nudity.

But the rules have led to fears that the site would no longer be able to accommodate what has become a thriving community of non-mainstream sexual expression. In 2015, Cosmopolitan magazine referred to Tumblr as a place where women "can explore their sexuality with the support of women without judgement or fear”.

In response to concerns, Tumblr wrote on its blog: "We fully recognise Tumblr’s special obligation to these communities and are committed to ensuring that our new policy on adult content does not silence the vital conversations that take place here every day."

It added: "Exploration of sexuality and gender, efforts to document the lives and challenges of those in the sex-worker industry, and posts with pictures, videos, and Gifs of gender-confirmation surgery are all examples of content that is not only permitted on Tumblr but actively encouraged.”

The changes come as Tumblr’s parent company, Oath, is undergoing a major reorganisation.

It had $4.6bn (£3.6bn) of its value written off by its owner, Verizon. The telecoms giant Verizon said it was laying off about 10,000 of its employees.