Killers who refuse to reveal where they hid a body could still be freed despite new laws aimed at denying them parole.
Parole Board chief executive Martin Jones said even when Helen’s Law starts next year the board was obliged to free inmates who pose no risk to the public.
Helen’s Law is named after Helen McCourt, from Merseyside, whose killer Ian Simms was freed from jail without disclosing the location of her remains.
Her mother Marie McCourt said the law could have “gone further”.
Mr Jones said prisoners would be questioned and failure to co-operate may not work in their favour.
But he said it was a “really difficult area” and the legislation does not simply mean “no body, no parole”.
Failure to co-operate and reveal such information is “frowned upon” by the board and could see a prisoner having requests for parole denied in the first instance, Mr Jones said.
However, he added: “What it cannot do is act as a complete block on your release.
“Ultimately if someone is no longer a risk, we must release them.”
Insurance clerk Helen McCourt vanished on her way home from work in 1988. Her killer was released from prison earlier this year.
Her 77-year-old mother, who is from St Helens, said it was “upsetting to hear the law may not have helped our case”.
She added: “Simms has a violent history. How can they say a man like that – who also won’t reveal information – is safe to be released?
“But they have to make sure Helen’s Law makes it harder and makes it far more difficult than it has been.”
Helen’s family spent five years calling for legislation to help give grieving relatives closure.
The Prisoners (Disclosure Of Information About Victims) Bill finally gained Royal Assent last month after a series of political and constitutional setbacks.
The law sets out to toughen up existing guidelines, making it a legal requirement for the Parole Board to take into account a killer’s failure to disclose the location of their victim’s remains when considering them for release.
It will also apply to paedophiles who refuse to identify those they abused.