A man who killed his ex-girlfriend in her mum’s home off Streatham High Road has become the first person convicted under double jeopardy law.

Mario Celaire, 31, today pleaded guilty to the manslaughter of Cassandra McDermott.

He had previously been found not guilty in February 2002 of her murder.

New evidence against him emerged after he admitted his guilt to an ex-girlfriend he also subjected to a vicious attack, and the Crown Prosecution Service (CPS) brought fresh charges against him.

His conviction is the first following a change in legislation enabling a person to be prosecuted again for serious offences such as murder, where compelling new evidence exists.

Cassandra, 19, was killed by Celaire in October 2001 in her mother’s house in Granville Gardens, Norbury, after an argument over her damaging a car he lent her.

She choked on the contents of her stomach as she lay unconscious after he punched her in the face.

The 31-year-old, who was living in a Croydon YMCA a the time, assaulted her after she returned home at 11.30pm on October 25 from using an internet cafe and picking up a takeaway in Streatham High Road.

She was discovered the next day by her sisters who visited the house, which Cassandra had been looking after for her mum who was on holiday, after they grew concerned she was not answering her phone.

Celaire’s conviction, six years and six months after he was initially found not guilty of the killing, began when he tried to murder an ex-girlfriend, Kara Hoyte, in 2007.

Celaire also pleaded guilty on Tuesday to attempting to murder Kara Hoyte, which could not be reported until today because of a court order.

Miss Hoyte - who was smashed three times on the left side of her head with a hammer by Celaire - recovered from her injuries and told officers, who were forming the case against her ex-boyfriend for her attempted murder, she had challenged Celaire - now calling himself Mario McNish - about what happened to Cassandra.

Miss Hoyte - who was partially paralysed for nine months - had found case papers relating to the murder trial in 2002 and wanted to know what happened.

He intitially told her Cassandra had died after being sick from eating a Chinese meal, but he finally admitted he lost his temper and punched her in the face before leaving.

As a result police launched a re-investigation of the original murder enquiry.

A police spokesman said the confession gave detectives 'new and compellable' evidence with which they could begin the process of having the original verdict overturned. They were also able to use bad character, which was not used during the 2002 trial.

Today Mario Celaire pleaded guilty to manslaughter of Cassandra and the attempted murder of Miss Hoyte.

He will be sentenced on July 3 at the Old Bailey.

Detective Chief Inspector Nick Scola, from the Homicide and Serious Crime Command who led the investigation into Cassandra's death, said: "I would like to pay tribute to two people.

"Firstly Cassandra's mother, Jennifer, was in court to finally hear Mario Celaire acknowledge his part in her daughter's death, seven and a half years after he struck the fatal blows.

"It is testament to her courage and determination in fighting for justice for Cassie that this day was reached.

"I would also like to pay tribute to Kara Hoyte whose bravery and determination to give evidence against Celaire, despite receiving near fatal head injuries, was astonishing. It is her strength that has helped bring justice to Cassie's family.

"Celaire was a dangerous man and this conviction again highlights the Met's resolve in tackling violent crime and using all the tools at our disposal to ensure that justice is done."