Hundreds of women have become pregnant after a long-term contraceptive implant failed.

They have also complained that they were left injured or scarred by the contraceptive rod inserted into their arm, which was supposed to protect them against conceiving for three years.

Implanon is a £90 device the size of a matchstick that is inserted under the skin of the upper arm. It gradually releases the hormone progesterone which stops the ovaries from releasing eggs and makes the womb less receptive to fertilised eggs.

The contraceptive implant is supposed to provide protection against pregnancy for up to three years and properly inserted it is 99.95 per cent effective among all users, compared with 99.7 per cent for the Pill.

But its failure is the worst ever to hit the NHS, forcing it to pay more than £200,000 in compensation to women hurt by the implants and those traumatised by unexpected pregnancy.

A total of 584 women who had the implant have reported unwanted pregnancies to the Medicines and Healthcare Regulatory Agency – the Government’s drugs and medical devices watchdog.

According to a lawyer, many of the women had suffered “psychological difficulties”, had miscarriages or decided to undergo abortions.

One woman who became pregnant and underwent an abortion said the trauma had led to her marriage ending.

The MHRA received 1,607 complaints about the contraceptive implant going wrong, some from doctors deeply concerned that the devices are difficult to insert and that it is impossible to check if they are correctly installed because they are invisible to X-rays.

Implanon’s manufacturer MSD, a subsidiary of global pharmaceutical giant Merck, has now replaced it with an updated product called Nexplanon, which has a new pre-loaded applicator and can be detected by X-ray or CT scan.

However, it has not recalled the existing stock of Implanon, meaning women are still being given it.

It was found in many cases that the implant failed to work because it was not released from the pre-loaded applicator and never inserted into the arm of the patient. In other women it was delivered too deep to work properly.