Seen arriving on the banks of the Thames in a black cab, Kathy (Gillian Taylforth) told ex-love Phil (Steve McFadden) she wanted to return to the Square - but he insisted she stay away.
Viewers had previously seen Phil picking up a passport on the Square in episodes aired earlier this week as he also made attempts to tell Ian something.
Last seen in 2000 before returning to South Africa, viewers had believed that Kathy was killed in a car crash.
Tonight's scenes have set up a huge storyline which will see Kathy return to Albert Square "later this year", the BBC has confirmed.
Talking about her return to EastEnders, Taylforth said: "When Dominic [Treadwell-Collins, executive producer] approached me with his plan, I was so shocked I got into my car and burst into tears! 'Kathy' has always been so close to my heart and it's absolutely wonderful to be returning to the show and reprising the role."
Adam Woodyatt, who plays Kathy's son Ian, added: "I'm thrilled, I couldn't be happier. I've finally got my mum back! I've been keeping this secret for so long and now I just can't wait to work with her again."
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Dominic Treadwell-Collins, executive producer, said: "I have always made my feelings on Kathy Beale and Gillian Taylforth very clear – she is part of EastEnders history, mother to Ian and Ben and one of the most important and iconic television characters on British television. Six months ago, we set ourselves the challenge of bringing Kathy back to the Square in a credible way. And I believe we have succeeded.
"Viewers will have to keep watching to find out exactly why Kathy 'died', where she has been all these years and why she now wants to come home to Albert Square. This is only the beginning of one of the most earth-shattering storylines EastEnders has ever seen…"
Here is a round up of Twitter reactions to Kathy's surprise return:
Kathy comes home! We are thrilled to welcome Gillian Taylforth back to #EastEnders #EELive pic.twitter.com/Cjm5CTa3ou
— EastEnders Press (@EastEndersPress) February 19, 2015
Kathy's back!!!! OMG!!!! I can't cope with much more #EELive
— Tracy Upton (@tracy_no_e) February 19, 2015
KATHY OMFG @dominictc YOU LEGEND #EELIVE
— Jake (@jaketague) February 19, 2015
Who thought we'd all be shocked BEFORE the killer reveal?
The return of Kathy =
#EELive #EastEnders
— BBC One (@BBCOne) February 19, 2015
Omg. I know it's absurd but give me Kathy's return PLEASE #EELive
— Ryan Love (@RyanJL) February 19, 2015
Dreams do come true #EELive pic.twitter.com/ayhBq9u6VM
— Adam Woodyatt (@AdamWoodyatt) February 19, 2015
Whoever made this is genius #Kathy #WutheringHeights #EELive pic.twitter.com/mBwkpKoy3I
— Tom Pilcher (@tompilch) February 19, 2015
