Some 11 million people tuned in for the first episode - up 700,000 on last year, according to overnight ratings.
It was the biggest overnight audience for any channel so far this year, excluding the World Cup, according to ITV.
It was also the fourth-largest audience for the launch of I’m A Celebrity in the show’s history.
This year’s opening episode saw the 10-strong crop of celebrities tackle skydiving, jungle creatures and a 100m (328ft) high challenge called The Edge - the highest the show has had so far, host Declan Donnelly told contestants.
The programme - which includes football boss Harry Redknapp, DIY SOS presenter Nick Knowles and ex-EastEnders star Rita Simons in the line-up - opened with Donnelly jokingly introducing Willoughby, who is standing in for his long-time co-host Ant McPartlin as he takes a continued break from his TV commitments.
The first four contestants seen in the show were entertainer and actor John Barrowman, Simons, The Vamps star James McVey and Coronation Street actress Sair Khan, who were met by the presenters on a yacht.
They were split into two teams - Barrowman and Khan in the red team, and Simons and McVey in the yellow team - before being whisked off in a helicopter to face the first challenge of the series.
Simons and McVey won the task, collecting 10 flags in the fastest time, and they chose The Chase’s Anne Hegerty and former X Factor singer Fleur East to join their team.
Redknapp and Knowles were left to join the red team.
A final challenge was won by the red team after they successfully picked up their jungle gear from a pontoon in the middle of a lake faster than their rivals.
They won the right to head to the more luxurious of two camps, Croc Creek, while the yellow team had to settle for the more basic Snake Rock, which has its beds on the jungle floor.
Actress Emily Atack will tackle Monday night’s first Bushtucker Trial of the series, called The Viper Pit.
The trial will see her having to collect stars for her campmates from chambers and holes occupied by snakes.
I’m A Celebrity ... Get Me Out Of Here! continues on ITV.
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