Computer firm Microsoft is hurrying to fix a bug with the Internet Explorer browser after it emerged that hackers have already exploited the glitch with attacks on US companies.
Microsoft said the bug could allow hackers to take over a computer, install malicious programmes and create user accounts.
Cybersecurity firm FireEye said so far there had been "targeted attacks seemingly against U.S.-based firms, currently tied to defence and financial sectors".
Several technology companies have urged the public to reset their passwords amid fears of a major security problem with a product used to protect people's personal data.
The Heartbleed bug affects OpenSSL, which many companies use to protect sensitive information, including people's password.
A small padlock icon appears on websites using OpenSSL to reassure users, but the loophole in the programme could have left it open to exploitation by hackers.
Blogging platform Tumblr posted a public notice about the bug, advising users to "take some time to change your passwords everywhere - especially your high-security services like email, file storage, and banking".
Finnish security company Codenomicon also said it would be "a good idea" to change potentially vulnerable passwords.
Some Army reservists will become specialists in areas such as cyber security, chemical and biological warfare and intelligence under plans to be outlined today, according to the Independent.
The newspaper reports that reservists will be able to take "enhanced training programmes" in these emerging areas as "an incentive to join and stay in the force".
It also reports that military planner believe that people who work in other professions - particularly in computing, sciences and languages - may already have skills that would be of use in these cutting-edge fields.
Cabinet Office Minister Francis Maude has warned that cyber attacks cost the UK economy billions of pounds each year.
Speaking on the launch of a new security cell to combat cyber attacks, he said the government intends to make Britain one of the safest places to do business in cyberspace.
Mr Maude said: "We know that cyber attacks are happening on an industrial scale and businesses are by far the biggest victims of cyber crime in terms of industrial espionage and intellectual property theft."
The government has formed a security cell comprising experts from MI5, GCHQ and the private sector to combat the growing threat from cyber attacks.
There will be around 12-15 analysts working for the so-called "fusion cell" from a secret location in London.
The cell is part of the Cyber Security Information Sharing Partnership, which has grown out of a pilot project last year whereby 80 companies shared information about cyber threats and methods of countering them.
Under the partnership, companies will have access to a web portal, described as being "like a secure Facebook", for sharing intelligence.
Cyber attacks are a growing problem, not just in terms of national security but also industrial espionage and intellectual property theft among British businesses.