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The father of Clare Wood, who died at the hands of a violent ex-boyfriend, said he was "quietly elated that common sense has come to the fore" after the introduction of what has become known as Clare's Law.
"I'm glad it escalated into what it has become," Michael Wood said.
Refuge's Isobel Shirlaw has told ITV news that the domestic violence charity has "grave concerns" over Clare's Law.
Shirlaw said that, while some people may "possibly" be saved from abuse by the law, "it doesn't really do anything to address the huge problem of domestic violence in this country".
Clare's Law was created after 36-year-old Clare Wood was murdered by a man who became known as the "Facebook fugitive".
In the days after her brutal killing, detectives issued a warning that her ex-boyfriend George Appleton might attempt to communicate with other women via the internet
Unbeknown to Miss Wood, Appleton had a history of violence towards women and was known to prowl online dating websites and Facebook in search of partners, often using different aliases.
The mother-of-one's body was discovered in the bedroom of her home in Salford, Greater Manchester, in February 2009. She had been strangled and set on fire.
Police watchdogs concluded afterwards that she had been badly let down by ''individual and systemic'' failures by Greater Manchester Police.
Today's national roll-out of 'Clare's Law' has been chose by the Home Secretary as it also coincided with International Women's Day and the launch of Domestic Violence Protection Orders.
DVPOs will enable police and magistrates' courts to provide protection to victims in the immediate aftermath of a domestic violence incident.
Mrs 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."
A father whose daughter was murdered by an ex-boyfriend with a secret violent past today said he was "absolutely delighted" women across the country have today been given the "right to know" their partner's history.
The scheme, known as Clare's Law 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.
Clare's Law gives women for the first time the right to know if a partner has a history of domestic violence and is being rolled out to police forces across England and Wales following a successful pilot scheme.
Clare 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 today: "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."
Latest ITV News reports
A father from Yorkshire whose daughter was murdered by an ex-boyfriend has welcomed a law allowing access to a partner's background.