"This was a thorough set of investigations which uncovered institutionalised failure within ITV that enabled the broadcaster to make money from misconduct on mass audience programmes," Ofcom chief executive, Ed Richards said.
The media regulator also said programme-makers showed "total disregard" for their own terms, conditions and broadcasting codes.
A separate report released by ITV revealed that The Catherine Tate Show was robbed of a prize at the 2005 British Comedy Awards.
Although Tate collected more votes for the People's Choice Award, Ant and Dec were announced as the winners for their Saturday Night Takeaway programme.
Three competitions on Ant & Dec's Saturday Night Takeaway were found to be in breach of the regulators' codes, including Jiggy Bank.
Entrants were selected purely based on where they lived, so many people ringing in wasted their money.
The ITV shows which brought the Ofcom fine
On Ant & Dec's Gameshow Marathon, winners were selected based on how entertaining and lively they sounded, and on Soapstar Superstar producers overrode the viewers' vote, so the wrong participants were put forward for eviction.
Ant & Dec said they were not aware of the phone-in scandals, despite being credited as executive producers on Saturday night Takeaway and Gameshow Marathon.
"Ofcom's announcement today is an appropriate moment to restate ITV's unreserved apology to the public for breaches that took place between 2003 and January 2007," executive chairman of ITV Michael Grade, said.
ITV made £7.8m from uncounted votes from the programmes in question and some 10 million telephone calls were affected.
The broadcaster pledged to give back the sum to charity and for viewer compensation, and this fact was taken into account by Ofcom in reaching its decision on the level of the fines.
The penalty is almost three times higher than the previous record of £2 million that was imposed on GMTV by Ofcom in September 2007.
BREAKDOWN OF ITV FINE
Ant & Dec's Saturday Night Takeaway - £3m
Ant & Dec's Gameshow Marathon - £1.2m
Soapstar Superstar - £1.2m
ITV2+1 - £275,000
"For a company that's delivering to its shareholders £150m - £200m a year of free cash flow, i.e profit, money, this is on the pathetic end of not very much," broadcasting analyst Steve Hewlett told the BBC News channel.
As well as the fine, ITV will have to broadcast a summary of Ofcom's findings on six separate occasions.
Five other programmes including I'm A Celebrity...Get Me Out of Here! were found to be in breach of Ofcom's broadcasting code, but no sanctions were imposed.
Ofcom also investigated allegations made about the last three series of X Factor, in particular the results of the 2007 finals, but found they did not breach the code.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.