Microsoft is to fix the security hole in its Internet Explorer browser that may have allowed Chinese hackers access to human rights activists' e-mail accounts.

In an unprecedented move, Microsoft, which normally issues such fixes on a regular monthly basis, said that the attention the problem had received forced it to move more quickly. This is despite it having issued a statement just a few days previously, in which it said there were “very few” infected sites on the internet.

The BBC reported that the action followed a decision by the French and German governments to advise citizens to abandon Microsoft in favour of other browsers.

If a web user were to visit a compromised site using a vulnerable browser, they could become infected with a “trojan horse”, allowing a hacker to take control of the computer and potentially steal sensitive information.

But security firm Sophos said now it had seen “copycat” sites trying to exploit the vulnerability.

“Though numbers are still very low, over the past 24 hours or so we have seen a few sites serving up malicious code attempting exploit the vulnerability,” it said in a blog post.

The bad publicity has allowed rivals such as Firefox to gain market share.

According to web analytics company StatCounter Firefox is now a close second to Internet Explorer (IE) in Europe, with 40 per cent of the market compared to Microsoft’s 45 per cent share.

In some markets, including Germany and Austria, Firefox has overtaken IE, the firm said.

“Given the significant level of attention this issue has generated, confusion about what customers can do to protect themselves and the escalating threat environment Microsoft will release a security update out-of-band for this vulnerability,” said general manager of Microsoft’s trustworthy computing security group, George Stathakopoulos.

“We take the decision to go out-of-band very seriously given the impact to customers, but we believe releasing an update is the right decision at this time,” he added.