"Hostile intelligence agencies" made hundreds of attempts to hack into the Treasury's computer system last year, Chancellor George Osborne has revealed.
Osborne said that the number of attempts averaged at least one attack a day.
"During 2010, hostile intelligence agencies made hundreds of serious and pre-planned attempts to break into the Treasury's computer system. It averaged out as more than one attempt per day," Osborne told delegates at the Google Zeitgeist conference in Hertfordshire.
"This makes the Treasury one of the most targeted departments across Whitehall," he said.
More than 20,000 malicious emails are sent to government networks "in any given month", he said.
He described one incident to delegates: "At some point last year, a perfectly legitimate G20-related email was sent to HM Treasury and some other international partners. Within minutes, it appeared that the email had been re-sent to the same distribution list.
"In fact, in the second email the legitimate attachment had been swapped for a file containing malicious code. Fortunately, our systems identified this attack and stopped it."