Nearly 600 women have become pregnant despite using a popular contraceptive implant, a health watchdog has said.
There have also been more than 1,600 reports of adverse reactions to the Implanon device, which is designed to prevent pregnancy for three years.
The NHS has been forced to pay compensation to several women because of the failures, Channel 4 News reported.
1.4 million women have used Implanon, according to the Department of Health.
The implant maker, MSD, said no contraceptive was 100% effective.
It added that unwanted pregnancies may occur if the implant was not correctly inserted, and said it had a failure rate of less than 1% if inserted correctly.
The Medicines and Healthcare Regulatory Agency said that since the launch of Implanon in the UK 11 years ago, 584 women using it had become pregnant, with 1,607 reports of adverse reactions.
According to the Department of Health around 1.4 million women have used Implanon since it was first licenced in 1999.
The implant is a small plastic rod which releases hormones into the bloodstream, and is inserted under the skin of a woman's arm by a nurse or doctor.
The MHRA said it had also received complaints from doctors and nurses about difficulties inserting the device.
Late last year Implanon was replaced with a device called Nexplanon, which is designed to be inserted more easily.
The MHRA says although the implant had been replaced, "the safety of Implanon remains under close review."
Nine of the 584 women who reported an unwanted pregnancy used the terms "device failure", "device dislocation", "device ineffective" and "device difficult to use" to describe their experience.
Others reported scarring and problems with removing the 40mm long implant.
A lawyer for some of the 14 women claiming for personal loss and damage said many had not realised the pre-loaded applicator had not released the implant.
Stephanie Prior, partner of Anthony Gold Solicitors, told Channel 4 News: "I have clients who fell pregnant as they were unaware that the Implanon device had not been inserted into their arm and they suffered psychological difficulties as a consequence of falling pregnant and later miscarrying or having to make the difficult decision to terminate the pregnancy."
The NHS has paid compensation to nine women who between them received £118,000.
In a statement, manufacturers MSD said: "The basis for successful use of Implanon is a correct and carefully performed subdermal insertion of the implant in accordance with the product instructions.
"If the implant is not inserted in accordance with the instructions and on the correct day, this may result in an unintended pregnancy. In addition, no contraceptive is 100% effective."
Correct insertion of the device could be an issue, according to the Faculty of Sexual and Reproductive Healthcare. Their vice-president, Dr Alyson Elliman said:
"With the older device, Implanon, there is a risk of non-insertion - when someone might think they have inserted it but in fact the implant is still sitting in the tube which then gets removed. But clinicians are also relying on women having accurate recall of their menstrual cycle, and whether they have already risked pregnancy during that cycle."
Ann Furedi, chief executive of the British Pregnancy Advisory Service, added: "Implants are an excellent and usually extremely reliable method of birth control. But all contraceptives have a failure rate, and although with implants this is tiny, women do need to be aware."
Advice from the British Pregnancy Advisory Service is that women should not panic, even if they have one of the old versions of Implanon. They said:
"As long as you can feel the device under your skin and you are within the three year time frame there is no reason to get it checked, and no reason to request the newer version. However if you cannot feel it, or if you are having any problems with it, do get it seen. An ultrasound or x-ray will quickly establish whether the device is there."