The four evicted Big Brother housemates who have the chance to return to the show are Grace Adams-Short, Lea Walker, Mikey Dalton and Nikki Grahame.
Nikki Grahame has been made the bookies' favourite to return
The contestants in the main house will choose one of the four - who are now next door - to rejoin them on Friday.
The twist has prompted almost 1,700 complaints, with viewers arguing they had already spent money on premium rate phone lines to evict the four.
Channel 4 is giving the £250,000 raised by the vote to charity.
The broadcaster has received about 500 complaints about the plan, with 1,200 more made to media watchdog Ofcom.
The four were picked from 10 former evictees in a public vote and the returning contestant will be eligible to win the show's £100,000 prize.
Surprised housemates watched on a television monitor as the four crowded into the Diary Room before entering the house next door.
Some 5.3 million viewers watched the live broadcast.
Meanwhile, either Imogen Thomas or Richard Newman will be evicted on Friday as each received three votes from housemates this week.
Sales adviser Jennie Corner, 18, had won immunity so she could not be nominated.
This means she is now guaranteed a place in the final on 18 August.