A plugin critical to the operation of thousands of websites was disabled for hours by Sky Broadband's parental content filters over Sunday night and Monday morning.

Access to the code.jquery.com website was blocked by Sky's content filters after the domain was mistakenly listed in the "malware and phishing" category. The site hosts code for jQuery, a code library which is used by over three-quarters of the top ten-thousand websites to ease the burden of creating responsive websites.

"JQuery was temporary blocked this morning having been misclassified. Our review process kicked in shortly afterwards and the site was unblocked just over an hour later," said Sky in a statement.

Without jQuery, sites like Google and The Guardian are unable to deliver the sort of instant response which internet users expect, and are forced to fall back to designs which require the entire page to be refreshed every time it is changes. The code required to enable those instant responses is usually hosted on one of three "content delivery networks", run by Google, Microsoft and jQuery itself. It was this latter CDN which was blocked by Sky.

It is unknown how jQuery ended up classified as malware, and for exactly how long it was blocked, although it was eventually removed from the list at 9:45 am. The Guardian understands that the most likely explanation is that an actual malware site was using code hosted on jQuery, causing the latter to be incorrectly categorised.

Due to the malware label, the network was even blocked for users who had selected "18+" on Sky's content filters. Only those users who opted out entirely could access a fully working internet.

The news adds a further twist to the debate over opt-in content filters. Despite being introduced by most major ISPs in the midst of a debate about access to porn and child protection, the filters block far more than just sexually explicit material.

For instance, Sky Broadband Shield's main filter also blocks sites classed as "dating", "cyber bullying" and "anonymisers, filesharing and hacking", while BT's parental controls offers parents the option of blocking "sex education" and "social networking" categories.

A number of sites have been incorrectly filed in these categories; in December, for example, an LGBT charity in the capital called London Friend was blocked by TalkTalk having been classified as "pornographic".

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