According to The Mirror today, the TV host made the decision after being informed that she would have a reduced role when the show is revamped later this year.
In a statement, Smith confirmed: "It's an ideal time for me to say goodbye to early mornings. Now that the format has changed I have taken the plunge and am moving on."
Smith's co-host John Stapleton has also been axed as a main presenter, but is to remain with GMTV under the title of 'special correspondent'. He will be seen presenting on-location reports from June.
Stapleton said: "After 12 years of getting up at 3.45am, I'm looking forward to having a few lie-ins a week."
It is thought that the shake-up has come due to budget cuts at GMTV. Smith and Stapleton currently present the programme's 'news hour' slot from 6am, but under the new arrangement, the show's main 7am hosts are expected to start work an hour earlier.