A court order to stop suspected stalkers under police investigation contacting victims could have made a "critical difference" to a woman killed by her ex-boyfriend, her father says.
Powers designed to help police act at "the earliest opportunity" come into force in England and Wales on Monday.
Those who breach the civil order could end up with five years in prison.
Clive Ruggles, whose daughter Alice was murdered, said the new orders "have teeth" but must be properly enforced.
From Monday, police will be able to apply to magistrates for a Stalking Protection Order (SPO), which will usually remain in place for two years.
Courts will also have the power to impose an interim SPO to provide immediate protection for victims while a decision is being made.
The orders will also be able to force stalkers to seek professional help.
Mr Ruggles, of the Alice Ruggles Trust, said the existence of the orders could have made a "critical difference" for his 24-year-old daughter, who was killed by her former partner.
Almost one in five women and almost one in 10 men aged 16 and over have experienced some form of stalking, according to the crime survey for England and Wales.
Stalking was made a specific criminal offence in England and Wales in 2012.
In Scotland, stalking is illegal under the Criminal Justice and Licensing (Scotland) Act 2010 and in Northern Ireland it is prohibited under the Protection from Harassment Order (NI) 1997.
Mr Ruggles said: "The new stalking orders have teeth, breaching them will be a criminal offence."
But he added they "absolutely" have to be properly applied.
Professor Jane Monckton-Smith, who specialises in researching homicide, stalking and coercive control at the University of Gloucestershire, said: "I think the orders could be really useful if they are used correctly".
But breaches could put victims in danger and must be taken seriously by the courts, she added. "Stalkers by their nature are obsessive and will keep going and going until they are stopped."
Plans to introduce the new civil orders were first floated in 2015, when Theresa May was home secretary.
In 2016, then-Home Secretary Amber Rudd promised to introduce them as soon as parliamentary time allowed.
Campaigner Sam Taylor, who runs a victim support group in Sussex, said the orders could give victims "respite" from being relentlessly pursued.
But she said they must be followed by a "significant investment in training" because there was still a "fundamental misunderstanding" in the criminal justice system of what stalking means.