Millions of computers with Intel chips will be slowed down as part of a fix for a security flaw that could leak sensitive information such as passwords and confidential files.

PCs and the servers used by major corporations are believed to be vulnerable to an exploit that could open a "persistent and undetectable backdoor into someone's computer," Mike Godfrey, cyber expert at Insinia Security told the Telegraph. 

The vulnerability, which reportedly went unnoticed by Intel for a decade, allows an outsider to gain total control of a computer by exploiting a core component, called the kernel, and disabling any antivirus security functions without the owner knowing. It could allow malicious software to steal passwords and sensitive files.

Intel chips appear in almost all personal computers and other technology. Financial institutions and businesses with large IT infrastructure may have been compromised for several years, Godfrey added. Train systems and autonomous cars also use the chips.   

Intel has reportedly warned software vendors including Microsoft and Amazon and Apple, who are believed to be creating a workaround to fix the flaw. But this fix could make computers 30 per cent slower, according to technology website The Register. 

"The real problem is for companies trying to support customers on their servers. Hypothetically, if a company once had capacity to support 100,000 users, that number may drop to 70,000," said Matthew Hickey, security expert and co-founder at My Hacker House. 

"It could have real cost implications for businesses that have been using or intend to use cloud technology and Intel servers".  

The bug has been known by security workers for some time, but is not due to be publicly disclosed until software giants like Amazon and Microsoft have issued a patch, so that the details cannot be exploited by criminals. Fixes are expected to be released in the next week.

Intel itself is unable to fix the flaw, so guarding against it requires a software update that will slow computers by between 5 per cent and 30 per cent, or to fix the processors completely.

Hundreds of millions of devices could be affected, including those still on the production lines. 

Mr Hickey added: "The real problems are for companies who are trying to get the best performance out of servers to support so many users. They may find that they had the capacity to support 100,000 users on their software, but that number could drop to 70,000. It could have real cost implications for business."

It is unclear whether anyone has been hacked thanks to this flaw, but penetration tester at Insinia Security, Matthew Carr, told the Telegraph that it was not inconceivable that a vulnerability that has existed for ten years had already been exploited by nation states, criminal gangs or expert level hackers. 

Intel have yet to comment on the matter.