Eggs, sperm and embryo samples of almost 2000 people are set to be lawfully destroyed as their time on ice expires.
A law passed in 2004 states that all frozen samples must be destroyed after 10 years, with the first deadline falling on November 22.
Families can apply to have the time lengthened but Fertility Associates have not been able to contact everyone who has frozen samples.
"This is urgent, this is within the week they need to make the decision," says Richard Fisher, the co-founder of Fertility Associates.
Around 300 people cannot be reached and another 650 have not replied to letters. There are fears that the unclaimed samples may be the only chance some have at raising children.
"What we don't want to do is for someone turn up on our doorstep in a year's time and say: 'I've come to collect my sperm' or 'I've come to collect my embryos' and we'll have to say: 'Sorry, they've been disposed of'," Mr Fisher says.