Speaking to his fellow housemates in the living room, the 28-year-old revealed that he felt "trapped" but insisted his decision had not been affected by others.
After leaving the Diary Room, he said: "I've got something massive to announce. I think I'm going tomorrow morning.
"I just don't want to be filmed here anymore. There's nothing any of you lot can do because I don't want any fuss."
The sales assistant added: "I've got massive cabin fever in here now, I don't like being trapped. I didn't realise how much I'd feel trapped.
"Some people when they're having a down day can sit in the background - I've never been able to do it. When big arguments happen, it's not worth me being here. I said from day one if I'm not having fun I won't stay.
"Two days ago I thought I was gonna walk out and I went out in the garden and I looked up and thought, 'I just want to be out there'."
Jay Mckray was most vocal in opposition, telling Mark that he will "regret" leaving the reality show.
Big Brother continues tomorrow at 10pm on Channel 5.
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Watch Mark announce his departure below: