Data from NHS Digital, which manages the register, implies that around 8500 women across England and Scotland have not been added to the register after they were fitted with an implant last year.
The register was recommended as a result of the breast implant scandal which saw some women fitted with faulty implants.
It records the details of every person who has breast implant surgery in case of a product recall or another safety concern.
But the latest figures suggest that many are being missed off the register and NHS Digital has asked women undergoing such treatment to ensure their surgeon has registered their details.
The figures show that operations on 10,500 people were recorded in 2020.
But officials have estimated this represents around 55% of the total number of cases.
This means that approximately 8485 people were not added.
The Breast and Cosmetic Implant Registry was launched in October 2016.
PIP (Poly Implant Prostheses) implants were withdrawn from the UK market in 2010 after it was found they had been filled with non-medical grade silicone intended for use in mattresses.
Estimates suggest that as many as 47,000 women in the UK have been fitted with the implants but the exact figure is not known.
A UK Government-ordered report in June 2012 found the implants were made from an unauthorised silicone filler and ruptured at twice the rate of other implants.
It was recommended that a national registry be created so that faulty implants could be efficiently tracked if products turn out to be faulty.
Since it was set up, around 60,000 breast implant patients have been added to the register.
Lee Martin, NHS breast surgeon and chairman of the Breast and Cosmetic Implant Registry steering group, said: "It is pleasing to see that the number of submissions to the registry has increased steadily over the past 5 years and now includes information from almost 60,000 patients.
"We want all patients, past and present, to be aware that the registry exists and ensure they are included by speaking to their surgeons.
"As a surgeon, I know first-hand the importance of patient information being included on the registry, it's crucial in providing good patient care and provides patients with assurance that they will be followed up with if there are issues with their breast implants in the future."
This article contains information from PA Media.