Declan Donnelly is "absolutely devastated" after Ant McPartlin returned to rehab this week.
Ant, who spoke out last year about battling addictions, is to get further treatment after his arrest over a suspected drink-drive crash on Sunday.
Saturday Night Takeaway's latest episode this weekend has already been cancelled, with Ant leaving the show for the rest of the series.
A spokesperson for Ant said: "Ant has decided to go back into treatment and step down from his current TV commitments.
“He’s spoken with Dec and ITV and asked for time off for the foreseeable future.
"As such Saturday Night Takeaway will not be going ahead this Saturday.”
Friends of the Geordie duo have described the events as "a bit of a bombshell for Dec."
Dec is said to be considering whether or not he will continue to work on TV without Ant, The Sun reports.
“He’s very upset and needs time to make any decisions, which is why this weekend’s episode of Saturday Night Takeaway was cancelled," the pal told the tabloid. “ITV understood that the show couldn’t go ahead without Dec and that he’ll have to make a final decision about what happens.”
A source added: “Dec is absolutely devastated of course, but he is also incredibly supportive.
“They are much more than co-stars. They’re best friends.”
In a statement, ITV said: “ITV has taken a joint decision with Ant and Dec’s team not to broadcast Saturday Night Takeaway this weekend.
“We will be reviewing options for the last two episodes of the series which would not feature Ant, who is taking time off to seek treatment. We very much hope he gets the help he needs.”
If Saturday Night Takeaway continues, Scarlett Moffatt is the current favourite to join Dec as a new co-host.