The purchase is dependent on regulatory approval from competition authorities and the consent of GMTV鈥檚 other major shareholder, The Walt Disney Company.
Chief executive at ITV, Charles Allen, said: 鈥淭his is another step in delivering our strategy to develop further the single ITV business.
鈥淲e have reached agreement with SMG on a number of points confirming the operation of the ITV Network, the digital transmission in Scotland of ITV channels including ITV2 and ITV News Channel and the sale of all SMG鈥檚 national television airtime through our single ITV sales house.
鈥淚TV plc鈥檚 acquisition of an increased stake in GMTV at a reasonable price will simplify the ownership structure and enable more agile decision making which will benefit both ITV and GMTV as we work even more closely together.鈥
SMG originally acquired a 20% share in the company in 1991, increasing to 25% four years ago.
The company made the decision to offload its interest in GMTV after deciding the ongoing investment was not 鈥渟trategic鈥 and disposal would best serve its shareholders.
Chief executive at SMG, Andrew Flanagan, said: 鈥淭he ITV merger created a 50% shareholder in GMTV and we don鈥檛 want to have a minority interest in someone else鈥檚 media business.
鈥淚t is clearly in our shareholders鈥 interests to sell the stake at an attractive price.
鈥淥ur focus is on driving and capitalising on the operational performance of our wholly-owned media assets as the advertising markets continue to improve.鈥