Opera singer Watson and veteran star of the stage Henshall were both pictured filming promotional material for the show earlier this month but were not named among the 10 celebrities in the show's launch lineup.
At the end of Tuesday night's episode, hosts Ant and Dec revealed that the campmates would be surprised with the news of the late arrivals during the latest trial.
The two new celebrities will have a chance to make a good first impression as they will be tasked with winning the stars for the evening meal for camp.
The group have a good track record so far, winning seven stars, nine stars and the full 10 stars in the three trials to date.
Henshall and Watson will make their debut in Wednesday night's edition of I'm A Celebrity... Get Me Out Of Here! on ITV. body check tags ::