TalkTalk says customers who have not yet chosen whether to activate net filters must opt out of its safety system if they wish to continue viewing adult material online.
The net provider will block access with pop-up boxes until decisions are made.
In a blog post, TalkTalk adds it will prompt customers to review their settings every year.
Sky announced last week that it would activate filters for inappropriate content by default.
BT and Virgin have yet to reveal any proposals for automatic filters.
Prime Minister David Cameron has previously called on ISPs to offer services with pre-activated filters in the interests of family safety.
TalkTalk customers are presented with information about the HomeSafe filter activation in their account settings pages.
TalkTalk says 95% of its 4.2m customers have already decided whether to activate the filters.
"We pre-tick the 'on' option, but it's the customer's choice," writes TalkTalk spokeswoman Alex Birtles on the firm's blog.
"Filters will only ever be applied if the customer has consented and they're able to change their mind or edit their level of protection at any point."
Those who have not yet visited the settings page will be confronted with a pop-up box if they try to access a web page that would be blocked by the filter, Ms Birtles adds.
'No silver bullet'
Like most filters, HomeSafe does not block material accessed via a web proxy or Virtual Private Network (VPN).
"There is no silver bullet when it comes to internet safety and we have always been clear that no solution can ever be 100%," said the firm on its website in a section for businesses who feel their sites have been unfairly blocked by the filter.
According to the website blocked.org, a project by the Open Rights Group (ORG), around 11% of the 100,000 top websites (according to Amazon-owned analytics firm Alexa) are currently blocked by default filters.
"Censorship should never be turned on by default," Jim Killock, executive director of ORG told the BBC last week.
"Filters block all kinds of websites, including some that provide useful advice to children and young people."