The premium rate phone watchdog has launched an investigation into Channel 4's decision to allow evicted housemates back on to Big Brother.
Former housemates find out on Friday who returns for the final week
Some 2,700 complaints have been sent to Icstis from viewers unhappy at being asked to vote again for contestants they thought had left for good.
The inquiry could result in Channel 4 refunding an estimated £3m to people who took part with phone or text votes.
Meanwhile, either Imogen Thomas or Richard Newman will leave on Friday.
They joined the four returning contestants - Grace Adams-Short, Lea Walker, Mikey Dalton and Nikki Graham - in a separate house next door following nominations.
Imogen is the runaway favourite to be evicted in the public vote, according to bookmaker William Hill.
It has given odds of 1/33 that she will go, compared to 10/1 for Richard.
On Friday evening, the housemate who survives the public vote will return to the main house with a former evictee.
The remaining housemates will have to choose which of the four they want to rejoin them in the house for the final week.
Nikki is the 1/4 favourite, with Lea at 11/4, Mikey at 16/1 and Grace - who is celebrating her 21st birthday - as the outsider on 20/1.
'Must not mislead'
Icstis is checking whether Big Brother breached its code of practice by asking viewers to vote for their return to the show.
This states that "services and promotional material must not mislead, or be likely to mislead, by inaccuracy, ambiguity, exaggeration, omission or otherwise".
"The majority of the complainants believe that they have been misled as they were under the impression that they had voted to evict the contestants permanently," the regulator's statement said.
Imogen is the overwhelming favourite to leave on Friday evening
It could decide to fine Big Brother's premium rate service providers up to £250,000 or bar their phone lines and text voting facilities.
Alternatively it may rule that guidelines have been followed and that there is no case to answer.
The Icstis investigation is expected to take no more than three months.
Channel 4 has defended the latest twist in the reality show's seventh series, saying all profits from the phone vote are being donated to charity.
"We welcome the investigation and will of course fully co-operate with Icstis in this matter," a Big Brother spokeswoman said.