A council that spied on a family to check they were living in the right school catchment area, has been criticised for its actions.
Jenny Paton, Tim Joyce and their three daughters were under covert surveillance by Poole Borough Council for three weeks in 2008.
They were tailed round the clock, secretly watched at home and their movements recorded in detailed surveillance forms.
The authority claimed it was acting under the Regulation of Investigatory Powers Act (RIPA). But a tribunal has found that it was not a proper purpose and nor was it necessary to use the powers.
Corinna Ferguson, legal officer for civil rights group Liberty, said: "Intrusive surveillance is vital to fighting terrorism and serious crime, but weak legal protections and petty abuses of power bring it into disrepute."
The tribunal also found that the surveillance breached the family's right to privacy under Article 8 of the Human Rights Act.
A Borough of Poole spokesman said: "The council accepts fully the ruling of the Investigatory Powers Tribunal and would like to apologise to Ms Paton and her family for any distress caused as a result of its actions in this case."
A Local Government Association spokesman said: "Councils need these powers to target serious criminals such as fly tippers, rogue traders and benefit fraudsters but they should also make sure these powers are used proportionally and the public have confidence in them."
The Government is reviewing the use of RIPA powers by local authorities as part of the Counter Terror Review announced by the Home Secretary last month.
The coalition agreement stated that the Government would limit local authority use of RIPA to "stopping serious crime" and only when approved by a magistrate.