Stalking and harrassment are not being investigated by police consistently or effectively, a report has found.
The inquiry into Sussex Police was ordered after the murder of 19-year-old Shana Grice, who was killed by her ex-boyfriend Michael Lane.
She reported him to officers five times in six months, but was fined for wasting police time. Lane was jailed for 25 years in March 2017 for her murder.
The report by Her Majesty's Inspectorate of Constabulary and Fire & Rescue Service (HMICFRS) made a string of recommendations on how Sussex Police could improve - the force has the second highest number of stalking offences in England and Wales.
Inspectors said some improvements had been made, but the report raised concerns no single definition for stalking had been adopted by police forces and government departments.
The report said: "As a result, police forces are not consistently identifying stalking, and are not protecting victims as a result."
It also called on the National Police Chiefs' Council (NPCC) to ensure forces around the country make improvements.
In the last two years reports of stalking and harassment cases have increased by more than 40% across England and Wales, HMICFRS said.
The report stated police forces are not using powers under stalking laws to search perpetrators' homes so investigations are "not as thorough as they could be" and victims of harassment are not being properly protected because injunctions are not being used.
There were also concerns over cases where victims were targeted online.
The report added: "We are concerned that Sussex Police's response to victims of stalking or harassment is not always as effective and consistent as it could be.
"This is because not all officers have received enhanced stalking training."
Not enough victims are being referred to specialised support services, the report said.
A training programme introduced after Miss Grice's murder to help staff better understand and identify stalking was "never fully completed" and most investigating officers had not received any training, according to the findings.
Lane pursued Miss Grice by fitting a tracker to her car, stole a house key to sneak into her room while she slept, and loitered outside her house. It later emerged 13 other women had reported him to police for stalking.
At Lane's sentencing, Mr Justice Nicholas Green said officers "jumped to conclusions" and "stereotyped" Miss Grice.
The force apologised for the way it handled the case and on Tuesday confirmed some officers would be facing disciplinary action over the death after the Independent of Police Conduct (IOPC) investigated 14 officers and staff.
However the family of Miss Grice said the changes made by the force are a "little, too late."
Miss Grice's parents, Sharon Grice and Richard Green, said: "Our daughter took her concerns to the police and instead of being protected was treated like a criminal. She paid for the police's lack of training, care and poor attitude with her life.
"It's only right that the police make changes, but it's too little, too late for Shana.
"Sussex Police should not be applauded for this.
"Instead we would encourage people to reflect on why they're making these changes. A young girl went to them for protection and ended up murdered in her own home by the very person she'd asked the police to protect her from."
Sussex police and crime commissioner Katy Bourne, who commissioned the report, said she hoped it would improve the force's response "dramatically" and scrutinise how other bodies were handling stalking.
It is the first time a commissioner has ordered HMICFRS to carry out such a report.
The force has been given three months to make improvements which include overhauling the risk assessment process, reminding officers of warrant powers under stalking laws and checking these are being used, making sure online crimes are being recorded correctly, and reviewing and improving training to make sure it is adequate.
The NPCC has been given up to six months to adopt a series of recommendations, including setting a definition of stalking to be used by all forces, ensuring crimes are properly recorded, reviewing the use of injunctions to protect harassment victims, and encouraging officers to make use of powers under stalking laws.
Here are a list of useful contacts for help and advice:
The National Stalking Helpline, run by Suzy Lamplugh Trust. Helpline on 0808 802 0300
Paladin NSAS (National Stalking Advocacy Service). Helpline:020 3866 4107
Women's Aid Federation of England. Helpline: 0808 2000 247
Victim Support – Stalking & Harassment. Supportline: 08 08 16 89 11
The Mix. Helpline: 0808 808 4994