Events are to come to a head in The Queen Vic, where Kat - played by Jesse Wallace - will reveal the identity of her mystery man in front of a room full of punters.
As her husband Alfie lunges for one of the Brannings - who earlier this month were revealed as the main suspects in the storyline - Kat will be forced to shout "You've got the wrong brother!" reports the Daily Star.
"It's the question that has been on everyone's lips for months - who is Kat's secret lover?" a show insider told the paper.
"The big revelation was always going to be in the Queen Vic in front of everyone and it will not disappoint. Jessie and Shane are both brilliant and this is one episode viewers will not want to miss.
"Alfie goes for one of the Branning brothers as he is convinced it's him who's been sleeping with his wife. Kat storms through the pub as he's got him on the floor and reveals the truth.
"We all got goosebumps as she yelled the line, 'You've got the wrong brother' and the comparisons were, of course, made to her showdown with Zoe. It really is classic 'Enders."
In 2001, Wallace took part in one of the most iconic scenes in the soap's recent history when she was involved in a showdown with her 'sister' Zoe Slater.
"You can't tell me what to do, you ain't my mother!" said Zoe (Michelle Ryan) during the row, before Kat dropped a bombshell by replying: "Yes I am!"
In recent weeks, viewers have seen Kat and Alfie try to repair their relationship, after Kat admitted cheating but said her husband did not know her lover.
When he discovers the truth, Alfie is initially devastated, but soon thinks he's worked out which Branning is involved. He plans to confront the culprit in The Queen Vic, but ends up targeting the wrong brother.
"It is down to Kat to reveal the truth," added the source. "Sadly it doesn't look like they'll be able to get over this one."
An EastEnders spokesman refused to confirm the reports, saying: "We do not comment on future storylines as we do not want to ruin it for our viewers."