Women and men across the country have for the first time been given the “right to know” if their partner has a violent past, as Clare’s Law was rolled out nationally.
The scheme, launched on Saturday, is named after Clare Wood, 36, who was strangled and set on fire by her ex-boyfriend George Appleton at her home in Salford, Greater Manchester.
Coinciding with International Women’s Day, Clare's Law is being introduced to police forces across England and Wales following a successful pilot scheme.
Ms Wood's father, Michael Brown, a retired prison officer from Batley, West Yorkshire, who spearheaded the "right to know" campaign after his daughter's murder in 2009, said: "I'm absolutely delighted.
"I must admit it's tinged with a bit of emotion and a bit of sadness but we have got what we were fighting for - to bring protection into the country for half the population."
Mr Brown, along with Salford MP Hazel Blears and Manchester radio station, Key 103, lobbied Home Secretary Theresa May to bring in the scheme.
Mr Brown, originally from Aberdeen, told Key 103: "I couldn't but thank Theresa May for taking the decision to roll the scheme out across the country.
"I would like to go back to the man that nobody knew, but I have comes down this path and I am delighted to have come out at the end of it, and I'm sure my daughter would be up there clapping."
Officially called the Domestic Violence Disclosure Scheme, the initiative is designed to provide victims with information that may protect them from a potentially abusive situation.
Clare’s Law allows the police to disclose information about a partner’s previous history of domestic violence or violent acts.
Ms Wood, a mother-of-one, had met Appleton on Facebook, and was unaware of his horrific history of violence against women, including repeated harassment, threats and the kidnapping at knifepoint of one of his ex-girlfriends.
He went on the run after killing Ms Wood and was dubbed the "Facebook Fugitive" before hanging himself while still at large.
Mr Brown added: "I can remember standing outside the coroner's office feeling lost....I'd lost a daughter and I thought I'd lost the battle. I wish I'd known what I know now because I felt desolate and for the pendulum to swing so far around, that has put a smile back on my face, it's hardly worth believing.
"It's there to be used. Get it used, ask! If you are in a domestic violence situation or you think you could be seek advice and get out of there, because the ultimate is 120 women a year have lost their lives, mostly at a young age."
The roll-out follows a 14-month pilot scheme in four police force areas, which provided more than 100 people with potentially life-saving information.
Saturday also marked the national launch of Domestic Violence Protection Orders (DVPOs). This new power will enable police and magistrates' courts to provide protection to victims in the immediate aftermath of a domestic violence incident.
Ms May said: "Domestic abuse shatters lives and this Government is working hard to provide police and local authorities with the tools they need to keep women and girls safe.
"Clare's Law and DVPOs are just two of a raft of measures we have introduced to hand control back to the victim by ensuring they can make informed decisions about their relationship and escape if necessary.
"Protection for victims is improving but sadly there are still too many cases where vulnerable people are let down.
"I am determined to see a society where violence against women and girls is not tolerated, where people speak out, and where no woman or girl has to suffer domestic abuse."
DVPOs can be used to provide immediate protection to a victim where there is not enough evidence to charge an alleged perpetrator and provide protection to victims via bail conditions.
They can last for up to 28 days, during which time the perpetrator can be prevented from having contact with the victim, giving them the opportunity to make decisions about their future safety with the help of a support agency.
Additional reporting by Press Association