In a damning report, the HMIC said police are failing victims of domestic violence right across England and Wales. Urgent reform is needed.
A father from Yorkshire whose daughter was murdered by an ex-boyfriend has welcomed a law allowing access to a partner's background.
More families are needing advice because their children are subject to child protection inquiries by social workers due to domestic abuse.
Home Secretary Theresa May has announced she will chair a new national monitoring group in response to one of the key recommendations made by the inspectors, to ensure every police force overhauls its approach to domestic violence.
"There needs to be action, and it needs to happen urgently," she told ITV News.
"What we see from this report is that the police have not been dealing with victims properly, they've not been dealing with domestic violence cases properly, and that needs to change," she added.
HM Chief Inspector of Constabulary Tom Winsor said: "Domestic abuse casts a terrible blight on the lives of very many people, and can have tragic consequences.
"In too many police forces we found there were serious weaknesses in services, which are putting victims at unnecessary and avoidable risk."
He added: "Domestic abuse is not only about violence, it is about fear, control and secrecy.
"It is essential that the police make substantial reforms to their handling of domestic abuse, including their understanding of the coercive and psychological nature of the crime as well as its physical manifestations."
Thousands of domestic violence victims are being failed by police forces across England and Wales due to "alarming and unacceptable weaknesses" in the way cases are investigated, inspectors have found.
In a damning report, Her Majesty's Inspectorate of Constabulary (HMIC) said only eight out of the 43 forces responded well to domestic abuse and the most vulnerable victims faced a "lottery" in the way their complaints were handled.
Poor attitudes, ineffective training and inadequate evidence-gathering were all heavily criticised by the watchdog, which has called for an urgent overhaul of the response to domestic abuse - from frontline officers up to police chiefs.
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."
The criminal justice system needs to focus on more than physical violence when pursuing a domestic violence case, experts said.
The chief executive of Women's Aid, Polly Neate, said in order to prevent the two deaths which occur every week at the hands of a controlling partner or ex-partner, authorities would need to take the psychological harm inflicted on the victim into account as well.
– Polly Neate
These survey results clearly reflect what our member services have been telling us for a long time: the criminal justice focus on individual incidents of physical violence cannot reflect the ongoing psychological harm caused by coercive control in intimate relationships.
We welcome the Government's recognition of coercive control in the Home Office definition of domestic violence...
Two women a week die at the hands of a partner or ex-partner; the next step to preventing these deaths is reform to allow the criminal justice system to take account of patterns of controlling and violent behaviour.