Ofcom says it is investigating a number of phone-in shows

It called for "rigorous" procedures to prevent the broadcast of "challenges that are almost impossible" and "inaccurate and/or misleading clues".

Trust could be lost between the broadcaster and audience, it warned.

Ofcom has launched an inquiry into the use of premium-rate phone lines on TV, while phone services regulator Icstis is investigating several programmes.

Ofcom says it currently has over 20 investigations underway into TV phone-ins but some of those relate to the same show.

An Ofcom statement said it was "essential that the demands of any competition on its audience are reasonable".

"We consider it necessary to remind broadcasters that they must take particular care in ensuring that there are rigorous compliance procedures in place," it said.

Code breaches

The regulator has just ruled against the Word Association show on Channel 4's now defunct Quiz Call channel.

Viewers were asked to name "things in Australia", and given the clue "Alice *******".

The presenter also indicated that the answer was fairly easy - before revealing the solution as Alice Springs Camel Cup.

Ofcom ruled this game was not conducted fairly and breached its Broadcasting Code

It also said the Quiz Call programme, made separately for Five, had breached its fairness code by describing a complex mathematical question in its Piggy Bank section as "very simple".

Icstis is investigating how phone lines were operated on 15 separate shows, including Channel 4's Richard and Judy and BBC One's Saturday Kitchen.