Liddiment, who witnessed the rise and fall of ITV's fortunes in his five years at the helm of ITV's programming output, is set to leave the Network Centre at the end of the year once a replacement has been found.
ITV has headhunters scouring the world of TV to find a candidate able to reverse ITV's dwindling audience share and advertising revenue.
During Liddiment's reign he commissioned successes ranging from Cold Feet to Who Wants to be a Millionaire?. However, he also faced criticism for commissioning the likes of Stars and their Doubles and introducing Premiership football to Saturday early evenings - a decision which was later reversed.
At this week's autumn schedule launch, ITV promised it would be investing £100 million of additional money into original programme commissioning in a bid to reverse its fortunes.
Liddiment is the third high-profile departure from ITV this year. The chief executive, Stuart Prebble, quit the network in May, while Leslie Hill, its chairman, retired in June.