Watch: Alison Hammond celebrates 20 years on This Morning
Hammond was led to believe she was filming another edition of the 'Dosh on the Doorstep' competition but Willoughby and Schofield told the viewers what was really happening.
They said: "We have made everything up because the person we are surprising today is actually Alison herself as she’s been on This Morning for an incredible and wonderful 20 years.
"We have set up a whole morning for surprise. Josie [Gibson] and Kate Lawler are in the house waiting to turn the tables on her."
When Hammond opened the door, she was met by fellow Big Brother alumni Gibson and Lawler who gave her a big red book and sped her off to the Midlands Arts Centre for a This is Your Life inspired recollection of her time on the daytime TV show with her family and friends in the audience.
After realising what was happening Hammond exclaimed: "This is the best day of my life!"
There were then a series of video messages from the likes of Hugh Jackman, Rick Astley, Davina McCall and Dermot O'Leary who paid tribute to her time on the show and shared some of their favourite moments of the actor and presenter.
At the end of the segment, Hammond said: “Thank you This Morning for giving me the opportunity firstly, I’m really overwhelmed by this to be honest with you. I’ve had the best 20 years. Every day has been the best and thank you to the viewers for watching. All I ever want to do is make you guys happy.”
Before joining This Morning in 2002, Hammond starred on the third series of Big Brother.
This Morning airs weekdays from 10am on ITV and ITV Hub.