The vast majority of new broadband customers in the UK are opting out of "child friendly" filters when prompted to install them by service providers.
The industry watchdog Ofcom found fewer than one in seven households installed the feature, which is offered by BT, Sky, TalkTalk and Virgin Media.
The filters block pornographic websites, as well as pages promoting self-harm or drug taking.
The default option was implemented at the behest of the UK government.
In July 2013, Prime Minister David Cameron announced the major UK internet service providers (ISPs) had agreed to offer "unavoidable choice" parental control filters, which block legal pornography and other adult subjects "by default".
New subscribers are offered the filter at the point of sign-up, and must actively choose to disable the parental control service.
However, the Ofcom report found users had overwhelmingly opted-out of the filter.
Of the four main ISPs, all of whom now offer a filter at the point of sign-up, TalkTalk was the only company to persuade more than 10% of people to subscribe.
The percentage of customers taking up the option for each service provider are as follows:
- Virgin Media - 4%
- BT - 5%
- Sky - 8%
- TalkTalk - 36%
All new subscribers to the ISPs were offered the "unavoidable choice" option, with the exception of Virgin Media, which only presented the feature to 35% of customers.
While BT and Sky launched filter services towards the end of 2013, TalkTalk's HomeSafe option has been in place since May 2011.
Virgin Media launched its filter, Web Safe, in February 2014, past the deadline set by the UK government.
The report also found that around 65% of new Virgin Media customers were not being offered the choice of family-friendly network level filtering, "primarily as a result of actions taken by installation engineers".
"The majority of new Virgin Media installations involve an engineer visit. Virgin Media believes that in many cases the engineer runs the broadband activation process and bypasses or ignores the filtering choice," Ofcom said.
"It has recognised that this is a failure in process and indicated it is taking steps to address this gap."
Tom Mockridge, Virgin Media's chief executive, said: "Ofcom's report clearly highlighted where Virgin Media has fallen short in meeting our original commitments.
"We take our responsibility to help families stay safe online very seriously and have taken immediate action to improve how we meet our commitments to government."