Government departments vital to the protection of the UK's critical national infrastructure are being attacked an average of 84 times a week by hackers.

The infrastructure consists of key departments like Government Communication Headquarters, the Ministry of Defence and the Home Office, along with power and communications companies in the private sector.

Electronic attacks on these organisations are recorded in confidence by the United Reporting and Alert Scheme. Between 1 January 1999 and 29 January 2002 government departments reported 13,146 hacking attempts.

Most proved unsuccessful with access being denied in 12,929 cases, but sensitive data was stolen or disclosed on 10 occasions and in 23 cases files were deleted or damaged.

The government was unable to say how many of the attacks were internal or external, citing the "disproportionate cost" of providing such a breakdown.

A Home Office spokeswoman said: "We continually monitor the threat of electronic attacks against the critical national infrastructure.

"We have well established defences in place but we are not complacent and keep it under constant review, particularly in the light of 11 September."