The blonde, who starred in hit U.K. soap opera Coronation Street for 12 years before quitting in February (12), showed fans an image of herself posing in a black bra while pointing a pistol at her head.
The photo, taken for a fashion shoot in October (12), was uploaded to Flanagan's Twitter.com account on Monday (17Dec12) with the message ""Head f**k"", just hours after the first victims of the atrocity were laid to rest.
However, on Wednesday (19Dec12), the actress hit back at editors of British newspaper The Sun for branding her ""brainless"" over the stunt.
In a series of posts on Twitter, Flanagan writes, ""I am absolutely disgusted in this (sic). Completely cruel. I will absolutely not stand for this kind of bullying. Anyone who knows me knows I have the biggest heart... I feel really chocked up (sic) that's so cruel.
""Honestly I've got tears streaming down my face how can you do that to someone over something so tragic. (I) feel sick can't believe I woke up to that. I've just been crying to my bf (boyfriend) before he left for work... Now I am just absolutely livid.""
The controversy comes amid growing pressure to tighten gun laws in the U.S. following the massacre on Friday (15Dec12) when Adam Lanza gunned down 20 young children and six adults at a school in Newtown, Connecticut before killing himself.