Britain's internal spy agency MI5 tapped innocent people's phone numbers in secret surveillance mix-up, according to a government report. Officers from the Security Service -- mainly concerned with the UK's internal security -- and Serious Organised Crime Agency mistakenly tapped the wrong telephones in as many as 30 cases, says the report.

However, on grounds of national security, none of the victims have been identified or told their phones were wrongly intercepted, the 'Daily Mail' reported.

But the report by the Interception of Communications Commissioner Sir Paul Kennedy has made it clear that the cases involved were only a fraction of thousands of interceptions authorised by British Ministers each year.

Sir Paul's report said that in January 2010 the Security Service made at least two errors "where an incorrect digit was used when warrantry paperwork was completed, due to human error".

It said: "This resulted in incorrect phone numbers being intercepted. On each occasion, the officer involved was briefed again on the importance of accuracy and cross-checking when completing warrantry applications."

MI5 also wrongly acquired data belonging to 134 phone numbers following a failure in software, the report added. "These errors were caused by a formatting fault on an electronic spreadsheet which altered the last three digits of each of the telephone numbers to '000'," the report said.

Sir Paul said that the intelligence agency destroyed the material and fixed the "technical fault". Search histories from another 927 internet connections were obtained by the Security Service without authorisation from sufficiently senior staff, according to the report.