But the implant's maker, Merck & Co., says the failure rate isn't exceptional, and that women should continue to use it.
Implanon contains synthetic progesterone and is inserted underneath the skin in the upper arm, where it is supposed to stop pregnancies for up to three years. More than 1 million implants have been sold since it was licensed in Britain in 1999.
However the Medicines and Healthcare Products Regulatory Agency said it had received more than 1,500 complaints of side effects including pain and scarring, linked to Implanon since 1999.
584 suffered unplanned pregnancies
Ronald Rogers, a spokesman for the New Jersey-based company, said the numbers of women in Britain who became pregnant were within the range the company would expect for that time period.
"We're very confident in the efficacy and safety of Implanon," Rogers said, adding no other countries where the implant is available have reported similar problems.
Rogers said 600 pregnancies among the 1 million women in the UK who use Implanon translates into a rate of less than 1%. No form of contraception is 100% effective, he added.
Britain's healthcare regulator said in a statement that Implanon is a safe, effective and reliable contraceptive "when used correctly."
The agency recommended that any women worried about their contraceptive implants should use a condom as well for extra protection or talk to their doctor.
(Sapa, January 2011)