Speaking at the launch for the duo’s new genealogy series Ant and Dec’s DNA Journey, Donnelly, 44, said he feared their double act would not “survive” a difficult period which saw McPartlin enter rehab after a drink driving conviction.
He admitted that he felt “incredibly angry” that his friend did not immediately confide in him.”
He said: “It has been a tough couple of years. It has really tested the bond we have shared since we were like 13 years old.
“I was incredibly angry at the start. So angry. Disappointed in him that he didn’t ever come to me and say: ‘I am struggling and I need you’.
“I thought that if the shoe was on the other foot I would have gone to him and said: ‘I am in trouble and I can’t handle this. I need your help’.
“And he never came to me. That hurt me a lot.”
Asked whether there was a point he thought he would lose McPartlin from his life, he replied: “Yeah. There was a point where I thought that was it.
“There were times I wasn’t sure it was going to survive, times you didn’t know he needed to survive or you wanted it to survive.”
McPartlin said he thought his career was over after the drink driving conviction and that the public had “had enough” of him.
The TV presenter, 43, took a break from the spotlight last year, in which he missed filming for Ant and Dec’s Saturday Night Takeaway and was replaced by Holly Willoughby as Donnelly’s co-presenter on I’m A Celebrity… Get Me Out Of Here.
He said the last two years had been “a very sad affair” but that “just being back on screen” was “quite emotional”.
He says: “Our friendship has been tested and we have come through the other end. And I will be forever grateful and thankful for the love we have.
“I thought the public had had enough.
“I just thought I wouldn’t be allowed to do what I love any more, which was a very sad affair.
“But, you know, coming back and just being back on screen and the affection and love and support that people have shown, has been quite overwhelming and very emotional.
“I can’t thank people enough.”
McPartlin appears alongside Donnelly in Ant and Dec’s DNA Journey, where the duo travel across the country, joined by genealogists and historians, as they follow their maternal and paternal bloodlines.
Filming for the programme began in 2017, but was halted during McPartlin’s career hiatus.
The series sees the Saturday Night Takeaway stars, both from Newcastle upon Tyne, journey to Ireland as well as to the US, where they visit the United Nations headquarters in New York.
Their research also unearths evidence that the bones of one of their ancestors were deemed “of historical importance”.
Ant and Dec’s DNA Journey starts on ITV on Sunday November 10 at 9pm.
With additional reporting by Press Association.