Council snooped on own staff with anti-terror powers.
New figures revealed the authority used the controversial Regulation of Investigative Powers Act (Ripa) on five employees between 2008 and last year. The legislation allows public bodies to snoop on phone and computer records, follow people and secretly film and record their movements.
One of the five workers was making a personal injury claim against the organisation but the council would not reveal details of the other four cases or say what measures were used to gather information.
Civil liberties group Big Brother Watch said there were questions to be answered about the authority's use of Ripa, which was designed to combat terrorism and other serious offences, for "relatively trivial matters".
Big Brother Watch director Nick Pickles said: "Birmingham City Council need to be transparent about why they felt it proportionate to use these surveillance powers for relatively trivial matters when neighbouring councils have used Ripa far less often."
Birmingham used the legislation 194 times in the three-year period in cases against alleged money launderers, fly tippers and benefit fraudsters.
A council spokesman said: "The use of Ripa is a last resort, and is only used when less intrusive methods of obtaining evidence can not be used.
"The council is recognised as an exemplar of national best practice in this area, using these powers only when necessary and proportionate.
"In order to be transparent, Birmingham City Council has placed its Ripa code of practice on the council website."