Carol, Sonia and Robbie head back to Albert Square in the early part of next year, much to the shock of their Walford-based relatives, who include Bianca (Patsy Palmer) and Branning brothers Max (Jake Wood) and Jack (Scott Maslen).
Coulson made her last appearance as Carol on October 7, 1999 when she left to care for her sick mother who subsequently died. Earlier that year, Carol made the shocking discovery that her daughter Bianca was having an affair with her then-partner Dan Sullivan (Craig Fairbrass) and a bitter showdown in The Queen Vic resulted in Bianca leaving Walford. Since parting ways, mother and daughter have barely spoken.
Speaking of her return, Coulson said: "I'm thrilled to be returning to EastEnders after all these years. I'm also excited and intrigued to find out what Carol and her family have been up to since we last saw her and where she will be going next."
Of her comeback, Cassidy stated: "To be invited back to EastEnders is such an honour and I am very, very excited that Sonia is coming back. For me, to be reunited with the original Jacksons is a dream come true and I think it will be a fantastic storyline."
Third returnee Gaffney initially left the BBC soap in February 2003 when his character Robbie migrated to India for a new life with girlfriend Nita Mistry (Bindya Solanki). He did, however, make a brief return in June 2004 for his half-sister Sonia's wedding to Martin.
Gaffney enthused: "I am really looking forward to returning to EastEnders for what looks set to be a really exciting storyline. I was thrilled when Diederick called me personally to ask for Robbie to return once more and working alongside Lindsey, Natalie and Patsy again makes it all the more special. I'm sure with the family being reunited it will mean that there must be a lot of drama in store."
A Walford source told DS: "Everything's being kept hush-hush about the Jacksons' return at the moment, but there are certainly a load of questions to be answered. Will the frosty relationship between Carol and Bianca thaw? Are Sonia and Martin still together? And how will Robbie react when he learns of his beloved Wellard's death? Everyone's so pleased to have them back."
Carol's youngest son - and the apple of her eye - Billie will also return to the Square at the same time. He was originally played by Devon Anderson but the character's final casting has yet to be confirmed.
Bosses have made no mention of remaining Jackson clan member Alan, played at the time by Howard Antony.
Coulson, Cassidy, Gaffney and the yet-to-be-cast Billie are due to begin filming in the next few weeks and will make their screen reappearance in the New Year.