US police have charged a former employee of mortgage giant Fannie Mae with computer intrusion after he allegedly installed a destructive virus onto the company's servers.

The affidavit accuses Babubhai Makwana of installing malware that was set to activate on 31 January 2009.

The virus would have taken a computer monitoring system offline, shut down access to the company's 4,000 servers and replaced all information with zeros.

"Had this malicious script executed [Fannie Mae] engineers expect that it would have caused millions of dollars worth of damage and reduced, if not shutdown, operations for at least a week," said FBI agent Jessica Nye in a sworn statement.

"The total damage would include cleaning out and restoring all 4,000 servers, restoring and securing the automation of mortgages, and restoring all data that was erased."

Makwana was fired by Fannie Mae on 24 October for changing a computer's settings without the authorisation of a supervisor. An administrator found the virus five days later and the police were called.

The server which contained the malware could only have been accessed by a few employees, and the police were able to trace the installation to an IP address used by Makwana's computer.

If found guilty Makwana faces up to 10 years in prison.