Aussie actor Ryan Moloney is the outsider who has been tipped to win the show at 33/1.
Moloney is up against British actress Tricia Penrose whose odds have been slashed after bookmakers received a string of bets backing the actress.
William Hill has seen several big bets placed on Penrose who played barmaid Gina Ward in Heartbeat.
She is now 2/1 to win – down from 40/1 in less than a week – and is seen as the main rival to 8/13 favourite Rylan Clark.
“Never before have we seen a Big Brother gamble like this,” William Hill spokesman Joe Crilly said.
“Last Wednesday, Tricia was 40/1 to win the show but now, after a sustained gamble, she is the second favourite and our best chance of getting Rylan beaten.”
Third favourites to win are US reality show couple Heidi and Spencer who are 10/1, with former Steps star Claire Richards and Moloney the 33/1 outsiders.