Thousands of women who have had breast implants have not had their details added to a national register set up in the wake of the PIP implant scandal.
The register, which is meant to track the safety of implants, was set up in 2016 following a government report that found as many as 47,00 women had PIP (Poly Implant Prostheses) implants filled with non-medical grade silicone.
It also found PIPs were twice as likely to rupture as other implants.
NHS digital, which manages the register, says 39% of NHS providers and 24% of private clinics may be putting women at risk by not adding their data.
A woman's details can only be added with their consent, so NHS Digital is now asking women who have implants to make sure their surgeon has registered the details.
Tom Denwood of NHS Digital said: "The development of the Breast and Cosmetic Implant Registry marked a major step forward in improving patient safety for those undergoing breast implant surgery.
"Now we are encouraging patients to ask their surgeon to submit their data to the registry and we hope this will lead to an increase in contributions.
"The more information that is contained within this important tool, the more it will benefit patients."
PIP breast implants were withdrawn from the UK in 2010 after it was found they had been fraudulently manufactured with unapproved silicone gel.
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In 2013 the founder of the company that made them, Jean-Claude Mas, was jailed for four years after being convicted of fraud.
It is estimated that 300,000 women in 65 countries received the faulty implants.