The move follows revelations in a BBC One 'Panorama' documentary, which presented evidence of what GMTV, which is 75% owned by the ITV network, termed "irregularities" in the system Opera was running.
The documentary claimed that viewers could have been cheated out of as much as £10m, over a period of four years. Shortlists which contained finalists for phone competition prizes had already been set before the lines closed, making it impossible for some viewers to even reach the shortlist stage, according the 'Panorama' allegations.
Phoneline competitions run by GMTV have been estimated as bringing in £45,000 a week for the channel and Opera.
The broadcaster, which is watched by 6m people every weekday morning, has said it is confident it has not breached any Ofcom or Icstis codes in its competitions, but conceded there have been "certain irregularities" in the way its phone operator, Opera, had managed its interactive services.
In a statement, Opera said that earlier this year it took steps to improve its administration procedures: "[We] can confirm that all competition contestant entries throughout the entire competition period are now considered equally and fairly, whether handled directly by Opera Interactive Technology or by its telecommunications partners, using a randomised computer programme.
"This process implemented as at 1st March 2007 has been observed and reviewed by Deloitte [ITV auditor] and has been successfully passed as satisfactory and in full compliance with the latest Icstis guidelines."
"Opera has in no way benefited financially from any errors in procedure in the past and will continue to keep its processes in line with any future Ofcom or Icstis guidelines."
A statement from GMTV said: "Our contract with Opera makes it quite clear that competition finalists could only be chosen after the lines were closed giving every viewer an opportunity of winning. It is now clear this detailed process was not adhered to by some Opera employees.
"We have therefore terminated our contract with Opera with immediate effect and our competitions will be suspended until further notice.
"Ictsis has launched an enquiry into Opera's systems. Panorama has said that as much as £40m could have been lost because of this. We believe the real figure, although substantial, will be considerably lower than this.
"We are determined to reimburse our viewers who believe they have lost out in entering our competitions and are urgently investigating ways in which we can do so."
GMTV is the latest broadcaster to be caught up in the phone votes scandal, which has blighted the broadcasting industry.
Ongoing action by the premium phoneline regulator Ictsis include an investigation into phoneline activity during the 'You Say, We Pay' section of the Richard & Judy show on Channel 4, with an outcome due in May this year.
The regulator could chose to levy a fine of up to £250,000 for any programme or channel proved to be benefiting from phone competitions that are unfair to the participating viewer.
Last week, ITV1's Saturday night talent show 'X Factor' was exonerated by premium-rate phoneline regulator Icstis, over an interactive votes irregularity in February, escaping a fine of up to £250,000. The broadcaster admitted that viewers voting for contestants in the last series of the 'X Factor' have been charged an extra 15p for each vote, when voting through the BSkyB platform..jpg)
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