A performance by Westlife on hit TV show The X Factor broke broadcasting rules because it could have triggered seizures, Ofcom ruled on Monday.
The chart-topping band appeared on the Saturday night programme in October with flashing laser effects which could have affected epilepsy sufferers and should have carried a warning.
Channel TV, which was meant to ensure the programme complied with broadcasting guidelines, admitted that the show did not meet the required standards.
However it said it had tightened up procedures since then.
In two short segments of the show, flashing was five times greater than the guidance given by regulator Ofcom.
Channel said its testing equipment did not spot any problems during rehearsals. But using different measurement methods, it agreed that two short sections of the actual live show - totalling three seconds - exceeded the limits.
Ofcom also pointed out there was a further risk to viewers because even the lower intensity flashing can cause problems if they last for more than five seconds and these ran for "relatively extended periods of time".
The regulator said the broadcast of the material breached its programme code.
© Press Association