HM Revenue & Customs put a stop to frauds totalling more than £103m in 2014-15, according to the Cabinet Office's UK Cyber Security Strategy 2011-2016 Annual Report (PDF).
The Cabinet Office said that government systems presented "an attractive target to cyber criminals", and that the threat had grown significantly over the life of the National Cyber Security Programme established in 2010.
HMRC took down almost 1,000 fraudulent websites in 2012, rising to more than 11,000 in 2015.
The department established a dedicated cyber security team in 2012 to secure government finances against cyber threats.
HMRC has set up a Cyber Security Command Centre to combat online fraud by correlating data across multiple sources to identify potentially anomalous or malicious behaviour.
Mike Hainey, head of analytics at HMRC, told Computing in 2013 that the organisation trawls the internet, including social media and other websites on which people share information, to find potential evidence of tax fraud that can be fed into its Connect data warehouse.
HMRC has also provided cyber security advice to taxpayers, including raising awareness of phishing attacks using emails purporting to come from HMRC. The department's cyber security pages were viewed more than 800,000 times between April and December 2015, the report said.
City of London Police's Action Fraud and National Fraud Intelligence Bureau said last month that nearly 100,000 people in the UK reported receiving phishing emails in 2015 at an average of 8,000 reports a month.
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