The TV duo appeared together in the audition episodes as they were recorded before McPartlin stepped back from his presenting commitments when he was charged with drink-driving earlier this year.
He was banned from the road for 20 months and fined £86,000 after pleading guilty to driving while more than twice the legal limit.
McPartlin, 42, also missed the final two episodes of his and Donnelly's Ant and Dec's Saturday Night Takeaway.
While Donnelly presented the final two episodes of Saturday Night Takeaway alone, Monday's first semi-final will be his solo debut on Britain's Got Talent.
The duo have fronted BGT since its inception in 2007 and have a golden buzzer option on the show which allows them to send an act directly to the competition's live semi-final.
The semi-finals will take place at the Hammersmith Apollo throughout next week before Sunday night's grand final.