The UK Treasury is being subjected to a cyber attack every day on average, according to Chancellor George Osborne.
He said that last year  "hostile intelligence agencies made hundreds of serious and pre-planned attempts to break into the Treasury's computer system".
He revealed the offensives in a major speech to Google's Zeitgeist conference yesterday and said this made the Treasury one of the most targeted departments in Whitehall.
Osborne said: "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. To the recipient it would have simply looked like the attachment had been sent twice.
"Fortunately, our systems identified this attack and stopped it."
Osborne hinted it was these attacks that helped persuade him to put the £650m aside for the National Cyber Security Programme announced during the Spending Review last year.
But he made it clear he had not been deterred, insisting: "We are determined to get the security question right, so that we can maximise the opportunities the internet age presents."
He said the government is re-examining its use of the internet to improve the delivery of public services and "opening up incredible new opportunities for progress".