Teenage girls in Bristol - some as young as 13 - will be offered contraceptive injections to curb underage pregnancy.

Ministers sent letters to 21 councils in teenage pregnancy "hot spots" - including Bristol - asking them to increase "long-acting" contraception.

This means more jabs - which can make girls infertile for up to three months - could be more readily available in school clinics.

Research carried out this year across 16 drop-in centres in city schools found that two percent of girls had already received the injections.

However, critics warned the move could give way to an increase in sexually transmitted diseases as girls shun condoms in favour of the jabs.

They also say it may promote promiscuity and health experts have voiced concerns that the injection is not suitable for girls who are still growing.

The controversial plans were revealed in response to a Freedom of Information request.

Letters demanded that council leaders and heads of primary care trusts take "urgent action" to make sure 2010 targets to reduce teenage pregnancy were met.

And ministers called for more school-based clinics to be set up alongside "an overall increase in the uptake of long-acting reversible contraception (LARC)".

Bristol has one of the highest rates of teenage pregnancy in the country despite a 2.6 per cent decline on last year.

Teenage pregnancy expert Anne Colquhoun confirmed the Bristol Primary Care Trust received the letter which highlighted Bristol as a "hot spot".

She added that there were plans to offer a full range of contraceptive methods in schools across Bristol.

"We want to make sure that every school health service is able to offer a full range of contraception.

"So yes, the injection will be a choice that young people in Bristol will offer. But we're not talking about a five minute decision here.

"We have trained members of staff discussing with girls whether they want to have sex at all.

"Staff will ask whether they are talking to their parents and if not they will be encouraged to where possible.

"They will also inform about preventing transmitted infections like. It's very rare that a 13-year-old will be having an injection as teenage pregnancies usually happen at around 16 or 17.

"However, if it did happen it would be discussed alongside the right child protection criteria."

But critics remained concerned the plans would promote underage sex and help spread sexually transmitted disease.

Tory leader Richard Eddy said: "We are all concerned by the number of teenage pregnancies but I'm not sure this form of contraceptive is going to solve the problem.

"What really worries me is the lack of parental consent and this method could well be giving the green light to a rise in sexual experimentation among young people.

"If the move goes ahead it will be a real threat to vastly rising levels of sexually transmitted diseases."

Last night the Department of Health said teenage girls will not be forced to have a contraceptive injection.

A statement said: "Thirteen-year-old girls will not be forced to have a contraceptive injection.

"The vast majority of 13-year-olds are not sexually active.

"Our teenage pregnancy strategy is about providing effective sex and relationships education to ensure young people have the skills to delay sex until they are ready, for parents to talk to their children about sex and relationships and access to effective contraception methods for those that are sexually active."

Britain has the highest rate of teenage pregnancies in Europe - second only to the US worldwide - despite a fall in numbers since the late 1990s.

In 2005, there were 39,804 conceptions by under-18s in England – a rate of 41.3 per thousand.