The troll accused the Smile singer of being responsible for the stillbirth, which happened in 2010 due to a viral infection, suggesting it was because she drank while pregnant.
The star, 28, tweeted: “Twitter makes it quite hard to report someone on here”, before posting a screen grab of the user’s comments along with: “There you go Twitter . Pls deal.”
One of the tweets the user sent was: “Remember when that drug addict Lily Allen dragged me for saying her son had Fetal Alcohol Condition.”
Mum-of-two Lily said of the attack: “I want him deleted because he’s talking about my son that died. Which isn’t funny, it’s devastating.”
The troll’s Twitter account is still active.
The verbal attack comes just days after trolls left Katie Price devastated by questioning the paternity of her latest child.
Lily, who’s never been afraid to speak her mind, is definitely no stranger to Twitter feuds. She’s famously had spats with Girls Aloud and Piers Morgan via the social networking site.
Last month she engaged in an online argument with Azealia Banks after the 212 singer called her husband and children ‘ugly’.