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Internet provider TalkTalk is to give customers the option of filtering malicious or offensive material out of their broadband service.

The company claims it is the first major ISP to offer centralised blocking at server level.

As well as stopping malicious software, parents will be able to prevent their children accessing adult material.

Security professionals have questioned whether the service will be able to react fast enough to new threats.

Conventional systems for blocking internet content rely on security software installed on users' PCs.

TalkTalk's 91Èȱ¬ Safe runs on the company's central computer system and sits between the web and individual home connections.

Its anti-virus system works by scanning a site that someone wants to visit to see if it harbours malicious programs. Those found to be clean will be put on a "white list" for 24 hours.

A spokesman for TalkTalk said that the system was discriminating enough to be able to block individual adverts on web pages that were booby-trapped with malware but would still let a user interact with the rest of that page.

To make the system work, TalkTalk has to scan all websites that its users visit. However the company said that it does not record details that could identify individual customers, such as their IP address.

Parents worried about their children seeing adult material will be able to log on to a web page and define their own content filters.

Sites containing pornography or online gambling can be blocked completely.

There will also be the option to put timed locks on certain websites, such as Facebook or game portals, to stop children viewing them when they should be doing school homework.

Similar PC-based systems have been criticised for their overly-broad filtering - often stopping young people accessing legitimate research sites.

TalkTalk said that its service would be able to tell the difference between sites that are wholly about a subject and ones that merely mentioned it.

For instance, parents who block sites that promote self harm might be happy for their children to see ones that educate about the issue.

Charlotte Nunes, a spokesperson for comparison site USwitch, called 91Èȱ¬ Safe was a "useful and well thought out freebie".

"Controlling security centrally from the network rather than on each individual device should make it far simpler for households to protect themselves against unwanted content," said Ms Nunes.

"The 'homework time' option is ingenious although not likely to be such a hit with the kids," she added.

Some security experts have questioned the ability of TalkTalk's system to protect against malware.

Rik Ferguson, senior security researcher at Trend Micro said that larger sites, frequently hit by threats, could present a problem.

He suggested that the 24-hour "all clear" white list might not work for services such as Facebook.

"The frequency with which we see rogue apps popping up on Facebook is much greater than one every 24 hours," he said.

Mr Ferguson suggested that a useful addition would be to scan outbound internet traffic, to spot when infected PCs are sending spam or taking part in large-scale web attacks.

TalkTalk's parental protection system would likely prove to be a small, but surmountable challenge to tech savvy teenagers, added Mr Ferguson.

He pointed out that many would resort to their mobiles in order to reach banned sites.

"There are a lot of ways around it," he said. "That's the big challenge for parents and security firms."

TalkTalk's spokesman insisted that its filters were not intended as a cure-all.

"This is the most robust system that's available but what it's not is a substitute for good parenting," he said.