High street shops and websites are marketing outfits at children that include Jack the Ripper, the possessed girl from horror movie The Exorcist, and a 'dead pet'.
The "inappropriate" costumes include a 'roadkill' bodysuit which is sold by Party Pieces, the company owned by Michael and Carole Middleton.
But a study conducted by parenting site ChannelMum.com found that nearly half of parents believe costumes aimed at kids are now 'too frightening'.
One in seven are worried that Halloween is becoming more sinister every year and a third (30 per cent) say costumes no longer have a traditional Halloween theme but are selected simply to scare.
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Among the nastiest Halloween trends this year are retailers pushing costumes for 18-rated movies aimed at junior aged children.
Outfits include Jason from the horror movie Friday the 13th aimed at eight year olds, and Regan from 70s demonic possession chiller The Exorcist, again an 18-certificate.
Also on sale are an outfit for kids as young as eight to dress as real-life Victorian serial killer Jack the Ripper.
And amid police warnings for pranksters to avoid joining the Killer Clown craze, sinister clown outfits are being targeted at children as young as five.
Worryingly, the survey of 1,434 mums by OnePoll found almost half (43 per cent) feel many costumes are 'too sexualised for young children', with revealing outfits offered for girls aged just four.
As a result, over two thirds of parents (68 per cent) now back age-appropriate ratings on costumes.
A further three quarters want retailers to take more responsibility for the outfits they sell, with 68 per cent claiming stores only care about profit.
One in 11 parents even admitted their child had had a negative experience wearing a Halloween costume - with 13 per cent targeted with sexual or inappropriate comments.
And half of children have been so terrified by another child's costume that they have left Halloween celebrations.
ChannelMum.com founder Siobhan Freegard said: "Halloween maybe the second biggest event for retailers after Christmas - but it should be about having harmless fun not encouraging children to wear horrible and even highly inappropriate outfits.
"Stores have become more responsible on everyday clothing, and dressing up should be no different. There is no excuse for putting profit before children's welfare."
Below are the eight most evil costumes on sale for children this year
1) Jason from 18-certificate movie Friday the 13th. Sold on eBay for children aged 8. 91% of parents would not let their child wear it.
2) The Slender Man. Fictional web character linked to stabbings and an attempted murder of a child in the US. Sold in Argos for 8 year olds.
96% of parents would not let their child wear it.
3) Dead Pet Roadkill. Marketed by Kate Middleton's family firm Party Pieces as a costume for 7 year olds.
93% of parents would not let their child wear it.
4) Serial killer Jack The Ripper. Sold for kids aged from 8 on Ebay and Amazon.
91% of parents would not let their child wear it.
5) Possessed Girl. Based on 18 cert film The Exorcist, for 7 year old girls. Sold by online site Joke.co.uk
91% of parents would not let their child wear it
6) Creepy Schoolgirl. Based on Wednesday Addams from the Addams Family. Sold for tots aged just 4 online.
90% of parents would not let their child wear it
7) Killer Creepy Clown. Sold for 5 year olds at Joke.co.uk
81% of parents would not let their child wear it
8) Miss Teen Hot Stuff. Sold by Fancy Dress Ball for 10-year-old girls.
94% of parents would not let their child wear it