The Government has no idea if a programme to drive down chlamydia infections is having an effect, MPs said today.
Inadequate measures are in place to check progress of the screening programme across health trusts in England, they said.
The latest report from the Public Accounts Committee comes after a damning National Audit Office study in November said millions of pounds of taxpayers' money has been wasted on the strategy.
It said the first four years were a waste of time because lessons were not learned on how best to implement the tests.
Chlamydia is the most common bacterial sexual infection and more than half of all new cases in 2008 were among under-25s.
If left untreated, it can lead women to suffer pelvic inflammatory disease, infertility and ectopic pregnancies.
Five years after the programme's launch, just 4.9 per cent of people aged 15 to 25 were being tested despite the target being 15 per cent.
It was only when the Department of Health forced primary care trusts (PCTs) to make it a priority that testing rates increased - to 15.9 per cent in 2008/09 against a 17 per cent target.
Today's report from MPs said: "Since the programme's launch an estimated £100 million has been spent but the department does not yet know what effect, if any, this has had on reducing the prevalence of the infection.
"The department's lack of urgency in pressing PCTs to reach a high volume of testing means that the programme has not yet reached the level of activity where models predict that the prevalence of chlamydia will be significantly reduced.
"As a result, more young people than necessary are still being infected and potential savings to the NHS in treating the consequences of chlamydia infection have been lost."
But a spokeswoman for the Department of Health said the programme had "improved dramatically" during the past year.
She added: "As a result, 38,185 young people were treated in 2008-09 and all local health trusts now offer screening compared to just a third in 2007."
She said a programme on this scale "takes time to perfect and improve".
The report noted that the cost of testing varies between health trusts and there is inefficient commissioning of services and equipment.
It said not all young people were being given advice on safer sex when they were tested for chlamydia despite instructions to health professionals to hold such discussions.
Tory MP Edward Leigh, chairman of the Public Accounts Committee, said: "This is a classic example of what can happen when the responsibility for delivering a national initiative is pushed down to local level, with little thought about the mechanisms and interventions needed at national and regional level to maintain efficiency and momentum."
Dr Peter Carter, general secretary of the Royal College of Nursing (RCN), said: "Screening is only part of the package and must be combined with effective awareness and prevention programmes in order to reduce the prevalence of sexually transmitted infections (STIs)."
Shadow health minister Anne Milton said: "Chlamydia screening is a crucial part of improving young people's sexual health but, at times when money is tight, it is shameful of the Government to come forward with such ill-conceived, ill thought-out and ineffective ideas."Reuse content