The 36-year-old said she planned to report the incident, which happened on a packed train from a day out in London, to police.
She wrote on Twitter: This vile excuse of a man just had his penis pushed up against me while on the overground train from London Bridge heading to Elmstead Woods.
Atomic Kitten Natasha Hamilton violated after man rubbed up on her on train
Furious: Atomic Kittens Natasha Hamilton took to Twitter on Wednesday to reveal she feels violated after a vile man rubbed his manhood against her on a packed train
I thought it was a bag until I felt him heavy breathing down my neck. I am appalled and calling the transport police when I get off!
Taking to the social media site a little later on, she wrote: Honesty I am shaking a feel so violated now! But more than anything I am so angry!!
I had the opportunity to take his picture before he left the train – this has to stop happening to people!
Cross: Natasha shared a picture of the alleged culprit on social media, writing about how upset she was
The British Transport Police told MailOnline in a statement: British Transport Police are aware of a report on social media of a sexual assault on a Overground train from London Bridge to Elmstead Woods yesterday (10/10/18).
We take every report of unwanted sexual behaviour with the utmost of seriousness – and through our Report It To Stop It campaign we are encouraging victims to come forward to tell us what happened.
You can do this by using our simple and discreet 61016 text service or by calling us on 0800 40 50 40.
Natasha later wrote: Taking to the social media site a little later on, she wrote: Honesty I am shaking a feel so violated now! But more than anything I am so angry!!
Thankfully, the rail network is a CCTV rich environment and we use every opportunity to use this crucial evidence to help prosecute suspects.
We also work tirelessly to combat sexual offences by running covert patrols across the rail network with plain clothed officers.
Her followers wrote: People like that need to be in prison,Im sorry Natasha to hear that happen to you I hope u filed a police report against such a disgusting low sick man,you will be strong Natasha the kittens and fans will stand by your side.
Surely that constitutes a sexual assault wether in or out of his trousers? hope youre ok btw as the shock of it all can hit u later xx. [sic].
Natasha got engaged to Charles Gay in November 2016, showing off her diamond sparkler on Instagram at the time, as she announced the happy news.
She wrote: After the most wonderful time in New York with the man I love, I was so high on life, little did I know he was planning something very special.
He proposed and I said yes! Ive had such an emotional rollercoaster few days @charlesgay Ill be stuck on cloud 9 for a long time…
The mother-of-four claims a man pushed his penis up against her while standing on a busy rush hour train