The National Crime Agency is warning computer users they have two weeks to protect against a "powerful computer attack".

Two pieces of malware software known as GOZeuS and CryptoLocker are at the centre of the alert.

People are being warned to make sure their security software is installed and up to date, and to run scans to check for any problems.

Important files should also be backed up, said the National Crime Agency (NCA).

The malware typically infects a computer via attachments or links in emails, said the NCA.

"(They) may look like they have been sent by genuine contacts and may purport to carry invoices, voicemail messages, or any file made to look innocuous.

"These emails are generated by other victims' computers, who do not realise they are infected, and are used to send mass emails creating more victims."

The agency said GOZeuS (also known as P2PZeuS) was responsible for fraud totaling hundreds of millions of pounds globally.

If a user clicks on an infected link or attachment, the malware silently monitors computer activity and tries to capture any private information, such as banking details.

The second threat comes from the Cryptolocker malware which can lock a user out of their files until a "ransom" of several hundred pounds is paid.

"Recent intelligence has suggested that more than 15,500 computers in the UK are currently infected, with many more potentially at risk," said the NCA.

FBI action in the US had weakened the network of global computers, said the agency, "meaning that action taken now to strengthen online safety can be particularly effective".