The pair had been for drinks at the Queen Vic with Lee's (played by Danny-Boy Hatchard) friend Beanbag when their troubles began.
Beanbag revealed to Lee that Whitney (Shona McGarty) had asked if he could help him get a job in security in Dover.
When he heard what his wife had been planning, Lee was furious and began to accuse her of flirting with his friend.
But, she insisted that she simply wanted him to help out more with their financial problems and be more like his father Mick (Danny Dyer), adding that he "didn't deserve her".
EastEnders: Whitney persuades husband Lee to go for drinks at the Queen Vic
EastEnders: Lee Carter was furious with Whitney
Well, I don't see Lee and Whitney lasting much longer!
Her comments certainly struck a chord with Lee, who turned around and slapped her across the face, leaving viewers stunned.
One fan tweeted: "No way did Lee just do that to Whitney #EastEnders." While another added: "Well that's Lee and Whitney's marriage finished #EastEnders."
Someone else continued: "Well, I don't see Lee and Whitney lasting much longer! #EastEnders."
Following the incident, Lee could be seen sobbing as he begged for forgiveness from Whitney while she lay curled up on their bed horrified by his violence.
EastEnders: Viewers were shocked when Lee hit his wife Whitney
EastEnders most memorable moments
Mon, January 23, 2017
From Wellard's tearjerking death to who shot Phil, here are the most memorable and shocking moments from Albert Square over the years.
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Martin Fowler got stuck underneath a double-decker bus that crashed into the market
EastEnders: Shona McGarty plays Whitney Carter in the BBC soap
The pair are set to try and save their marriage on Valentine's Day but will Whitney really be able to forgive Lee? And will she keep his violent outburst a secret?
EastEnders continues on Monday at 8pm on BBC One.
If you've been affected by the storyline in tonight's episode of EastEnders, support is available from the Samaritans at samaritans.org or on 116 123.