Bosses on the Channel 5 show have decided to make life hard for some of the contestants, with half spending their time in the grotty basement area, where there is no running water, no windows and residents will have to scavenge for food from the scraps from the main house.
Told their task by Big Brother in the Diary Room, Dettori and Clark looked distinctly uncomfortable but said: "Somebody's got to do it."
As the other housemates were introduced in pairs by host Brian Dowling, Dettori and Clark had to decide who would go the luxury house, and who to the basement.
Hamilton took it well, but laughed bitterly when she saw conditions in the basement, saying: "Oh my God, charming, delicious, oh, now I feel at home."
She added: "We're all in this together" - which was daubed on the wall.
Next Dettori and Clark had to decide who was more "fake" out of Neighbours actor Ryan Maloney, who has played Toadfish in the Australian soap since 1995, and ex- EastEnders and Footballers Wives star Gillian Taylforth, and sent Maloney to the basement.
He said: "I'm up for a challenge, bring it on."
In the basement he and Hamilton heard an announcement about a champagne reception upstairs. Hamilton said: "We're in the sin bin."
And Maloney added: "It looks like we'll be here all night."
Clark has been installed as favourite to win the show by bookmaker William Hill, with Dettori next.
The contestants will spend the next few days holed up in the custom-built property in full glare of TV cameras with a winner emerging later this month if they can prove popular enough with viewers and their own housemates. The series is expected to last three weeks, with the first eviction due on Wednesday.
Dettori enters the house just weeks after he was given a six-month ban from his sport after a positive drugs test, which he said had left him feeling "ashamed and embarrassed".
"It doesn't matter what I took or how much - what I can say is it didn't make me ride any better or make my horse run any faster," the Italian-born showman said after the ban, which followed a positive test in Paris in September.
Hamilton found fame after starring in a VW ad campaign, but went on to have a problem with drink and drugs. She dated music and TV mogul Simon Cowell when she was in her teens.
Dettori and Clark then had to decide who fancied themselves more, former Coronation Street actor Sam Robertson or glamour model Lacey Banghard.
Apologising as usual to the loser in their decision, they said: "Sam, sorry pal."
Robertson, who was wearing a sweater and a kilt, kissed Banghard on the cheek before heading off to the basement.
The next pair were former Steps star Claire Richards and former footballer Neil "Razor" Ruddock, and Clark and Dettori were told to choose which would have the bigger ego - to be sent to the basement.
Clark said: "I'm sure that Neil Ruddock is a lovely man, but, my God, that's Claire from Steps."
Dettori said: "I met him in his heyday, he had a big ego, I think Razor can handle the down."
Dowling asked Ruddock: "You and Frankie have some history? Can you fill us in?"
Ruddock quipped: "He will get filled in, and yes, we've got a bit of history."
Dettori and Clark were told: "Big Brother may be cold hearted, but is not cruel enough to split up a young married couple. Will it be those two or you two that go to the basement?"
Clark said: "I'm really sorry, you live in LA, I come from Essex" and Dettori added: "Yes, I really feel the cold."
Pratt said: "I'm fine to be in the basement, then I don't have to be with them."
The series of Celebrity Big Brother is the fourth to be screened by Channel 5 in the space of 18 months, after resurrecting the show which had been dumped by Channel 4.
Ratings have been much reduced for the programme since it left Channel 4. The most recent CBB series which finished in September and was won by Julian Clary drew an average of just 2.3 million viewers, the lowest audience yet.
It was almost three million down on the 5.2 million average audience for the first ever series in March 2001.