Family planning groups and the Department of Health yesterday rejected newspaper allegations that 400 girls aged under 16 using a contraceptive implant have been "sterilised".

The revelation that the girls were using the implants which keep them infertile for three years ignited fresh controversy over the government's strategy to bring down teenage pregnancies - as well as the right of parents to know what is happening to their children.

Family planning campaigners pointed out that the implants could be removed at any time with almost immediate return of fertility. But some parents will be distressed, not least because contraceptive implants have proved controversial.

The figures came from a parliamentary answer to Tim Loughton, the shadow children's minister. Although 400 is not many in the context of the 7,617 girls under 16 who became pregnant in 2000, he told the Sunday Times he was concerned that the use of implants would encourage promiscuity and spread sexually transmitted diseases.

A further 2,500 girls aged 15 or younger are having three-monthly contraceptive injections.

"This will not help young people have a more responsible attitude to sex," Mr Loughton said. "It is no good making sure girls do not become pregnant if they are going to sleep with numerous partners and get infections."

But family planning experts say doctors are well-advised to suggest the implants or injections because they are safe and effective and sexually active girls under 16 will not want to become pregnant for some years.

"We wouldn't be at all concerned about it," a spokeswoman for the Family Planning Association said. "It [the implant] is highly regarded and perfectly safe."

If the implants were not widely used by older women, she said, that was probably because their GPs had offered only the oral contraceptive pill. But a wide variety of contraceptive options now existed, and doctors should discuss all of them with their patients.

Young, sexually active women should bear in mind the risk of sexually transmitted infections and use condoms as well, she said.

But many doctors will be offering a three-year implant or three-monthly injection because they fear younger girls will not remember to take the pill every day, or may fail to take it because of the difficulties in concealing packets from their parents.

Under such circumstances, it is unlikely they will manage to keep a supply of condoms with them.

While the implants have been licensed for use in the UK after clinical trials that show they are safe, it is unlikely that they have been tested on girls under the age of 16. The implants, the size of a matchstick, are inserted under the skin of the upper arm and are not visible. They are designed to release a dose of 40 micro grams of progestogen a day, reducing ovulation and making it less likely that an egg will be fertilised.

The implant, Implanon, made by Organon, is effective and considered safe and acceptable. However an earlier version lasting five years, Norplant, which was distributed in the UK by Hoechst Marion Roussel, was withdrawn in 1999 because of complaints about side-effects. Hoechst Marion Roussel insisted these were unfounded and that it had discontinued the implant because of "trial by media", but the furore testifies to some public discomfort over the idea of a long-term hormone-releasing implant.

Teenage pregnancy rates in the UK are higher than in the rest of Europe. A comprehensive strategy, launched after a report from the Social Exclusion Unit, is bringing the numbers down, but too slowly to meet the aim of a 50% reduction of under-18 pregnancies by 2010.

A spokesman for the teenage pregnancy unit said the method of contraception used was a matter for the individual, not government.

The object of the campaign was to get information to those individuals to allow them to decide whether and what contraception to use. The campaign aimed to get "good, sensible guidance to children", based on research that showed that early information from schools and parents led not only to a fall in teen pregnancies but also to children becoming sexually active at a later age.