South Wales police have apologised for their handling of the case of a woman who was taken to hospital following a brutal hammer attack by her partner days after she had complained to officers about a previous assault.

The Independent Police Complaints Commission (IPCC) will on Wednesday criticise the handling of the case by the force, which failed to take action against the convicted rapist Christopher Veal despite his history of violent attacks on women and even though he was on licence from prison.

Veal moved in with the woman in Cardiff on his release from jail without her knowing that he had been in prison for rape. She went to a police station after he assaulted her but was sent home by officers.

Days later Veal assaulted the woman with a claw hammer in front of her young daughter and son.

Only then was he was arrested and recalled to prison, where he later killed himself. The South Wales force did not refer the case to the IPCC – as it was required to do – and the watchdog only learned about it when an MP took up her complaint.

The IPCC has been examining why the officers did not appear to make even cursory checks that would have identified the suspect as a violent serial sex offender who had been in prison for rape. It is expected to release its lengthy report on the case on Wednesday.

Deputy Chief Constable Matt Jukes of South Wales police said: "It is clear that we did not provide the victim in this case and her children with the care and protection that she needed at a critical time and we are sincerely sorry for that.

"Over recent years we have worked incredibly hard to improve our response to incidents of domestic abuse and, whilst this has come too late to support the victim in this case, we have good evidence that it is providing a much-improved response to victims of domestic abuse now, helping to keep them safe."

An officer is to face misconduct proceedings, and what the police call "performance failings" by another officer and three staff members are being addressed.

In her first interview, the woman told Channel 4 News she felt ignored by the police. "I trusted the police to help me," she said. "They knew he was a rapist … and they sent me home.

"I trusted them to look after me and my children. I've been let down. My children have been let down. They've seen some horror film which no children should have ever seen."

She recalled a "massive bang" as Veal burst through the front door. "There was my partner with a claw hammer. He said: 'Come here you bitch' and he smashed me with a hammer.

"He kept hitting me. I tried to run out on to the veranda to scream … and whilst he told my daughter to stop her brother from screaming he carried on hitting me … I remember him standing behind me and the hammer coming in the front of my face and knocking my teeth out." She lost consciousness and woke with serious head and facial injuries.

Last year the Guardian revealed that South Wales police were at the centre of six investigations into serious failings in the handling of domestic violence, rape and sexual assault complaints including the Veal case.

Chief constables across Wales and all four police and crime commissioners were asked to attend meetings with the IPCC over concerns that basic errors were being made when dealing with allegations of sexual violence.