The Storm worm is showing no signs of letting up in its Valentine's Day assault.

Researchers from major security firms have uncovered thousands of spam emails spreading the Trojan in the days leading up to 14 February.

The worm has been spreading since January in the form of email greeting cards with subjects as 'You Stay in My Heart' and 'Thinking of U All Day'.

Users are taken to a fake e-card site and asked to download an application called 'valentine.exe'. The executable file is a Trojan which will add the user's computer to the huge Storm botnet.

The botnet has become one of the most formidable security threats on the internet, and researchers suspect that Storm's creators are now renting out the infected machines.

Storm has used fake holiday email greeting cards as a way for spreading the infection since it emerged in early 2007.

Symantec researcher Silas Barnes noted in a company blog that Storm is hardly the first security threat to pose as a holiday greeting.

"Holiday theme-based threats have enjoyed great success since at least 2000, and every year attackers are successful in compromising machines via constructed email messages," Barnes wrote.

"While the messages change from year to year, the mitigation strategy remains the same, which includes keeping antivirus signatures up-to-date and treating all emails carefully."