Oxfordshire County Council officers used “spy” powers to catch out conmen and pubs and shops selling to underage children at least 20 times in nine months, new figures show.

They launched the operations under the Regulation of Investigatory Powers Act (RIPA) against rogue traders and shops selling knives, fireworks and alcohol to children.

They uncovered 18 premises selling underage goods to children, and saw 44 pass the tests.

Those which failed included three stalls at car boot sales held at the Kassam Stadium in Oxford on May 31 this year, when knives were sold.

Two were warned by Oxfordshire County Council trading standards officers and one was cautioned by police.

Another operation on December 8 in Oxford found three shops out of 10 tested sold cigarettes to under-18s.

And hidden cameras in Goring caught rogue traders demanding payment for work, leading directly to their arrest.

The 20 uses of the act were from October 2010 to June 2011 and disclosed in a new council report.

Further use of the act was not disclosed because of impending legal action.

The act has hit headlines with cases of dog owners being watched over fouling concerns and the monitoring of parents to see if they lived in a school catchment area.

But the Office of Surveillance commissioners praised Oxfordshire’s used of RIPA in a May inspection as a “Rolls Royce” set-up, with good policies and training to avoid misuse.

The powers have been used by County Hall 69 times over the last three years and 266 times between 2001 and 2011.

All but six were for trading standards and these included surveillance to gather evidence over childcare proceedings.

Judy Heathcoat, council cabinet member for safer and stronger communities, said: “We had a very good inspection report, I’m pleased with the way it is being used. We implement the act in the way it is meant to be implemented.”