Derek, played by Jamie Foreman, is one of three suspects still left in the running in the ongoing storyline - remaining in the frame alongside his brothers Jack and Max.
Digital Spy opened up a new poll after Michael Moon was eliminated on Friday evening, and more than 30,000 votes have already poured in from fans who wanted to have their say.
The current results show that almost half of respondents believe that Derek is the mystery man in question, as he has picked up 48.8% of the votes.
Max (Jake Wood) is in second place with 33.2%, while previous reader favourite Jack (Scott Maslen) is now the outsider and has the remaining 18.0%.
While Max is currently concealing a seemingly-unrelated secret from fiancée Tanya Cross, it seems that most viewers do not believe that he has also been hiding an affair with Kat.
Jack's new romance story with Sharon Rickman also appears to have established him as a less likely suspect in the eyes of fans.
Kat (Jessie Wallace) is currently away on a marriage-saving holiday with Alfie (Shane Richie) after confessing to her affair. The mystery continues when the couple return to screens in a fortnight's time.