Horrified witnesses looked on as University of Kent student Molly McLaren was savagely attacked at the Dockside outlet centre in Chatham, Kent.
Her boyfriend Joshua Stimpson, 25, was arrested at the scene and has now been charged with her murder.
Miss McLaren was sitting in the passenger seat of a black Citroen C1 when she was attacked around 11am yesterday.
Witnesses say her attacker trapped in her car then slashed her throat as she beeped her horn and screamed for help.
Police said she sustained injuries consistent with a stabbing and died at the scene.
Miss McLaren and Stimpson had confirmed their relationship on Facebook last December and their social media profiles featured several pictures of them together.
Kent Police have now referred the murder to the Independent Police Complaints Commission, after Miss McLaren may have previously reported Stimpson to the police.