The national screening programme for chlamydia is not backed by evidence it is beneficial, say researchers.
Chlamydia is the most common sexually transmitted infection
Opportunistic screening for men and women under 25 is currently being rolled out across England.
But in the British Medical Journal, Dr Nicola Low said further randomised controlled trials were needed.
The Department of Health said opportunistic screening was the only kind so far backed by the World Health Organization.
Chlamydia is the most common sexually transmitted infection in the UK, with studies suggesting one in eight men and one in 10 women carry the infection.
The government has been rolling out an opportunistic screening programme in which people are offered a test for chlamydia when attending their GP surgery or genito-urinary medicine (GUM) clinic.
Two pilot studies were done in GP practices and a screening rate of 50% was achieved.
'Need to find evidence'
Dr Nicola Low, an epidemiologist at the University of Berne in Switzerland, said there had been "uncritical acceptance" by the National Screening Committee of reduced rates of chlamydia in countries with screening programmes - such as the US and Sweden - without proper evidence of the benefits and harms.
Between 10% and 40% of infected women who are left untreated develop pelvic inflammatory disease (PID).
If PID is not treated, it can lead to chronic pelvic pain, ectopic pregnancy and infertility.
Trials of proactive screening - where people are invited to be tested on a regular basis - have shown it can cut rates of PID by half.
But Dr Low said it was not clear if opportunistic screening would produce the same benefits.
"I'm suggesting that we need to find the evidence to support what we're doing," she said. "I think it was introduced before we had a proper understanding."
Dr Low said someone who received a negative result under the opportunistic screening programme might be given a false sense of security, when people actually needed to be tested regularly.
And unlike in chlamydia testing pilot schemes, GPs in the national programme do not have to participate and are not paid for the work.
In an accompanying editorial, Dr Rachael Jones, consultant in GUM at Chelsea and Westminster NHS Foundation Trust, said rates of infection were continuing to increase.
"Most people who are affected are unlikely to seek sexual health testing and may only be assessed via a proactive approach rather than the opportunistic screening programme currently offered," she said.
Dr Richard Ma, a GP in Islington and member of the Royal College of GPs' sex, drugs and HIV task group said: "In England we had to take the pragmatic approach.
"There are weaknesses with opportunistic screening, but when it started there was no way to get all GPs involved. They want to do proactive screening but there is no funding."
He added: "Even finding a case of someone who has chlamydia who wouldn't have otherwise known is very positive, and on that basis the programme is a success."
'Burden of infection'
A Department of Health spokesman said opportunistic screening remained the only strategy recommended by WHO for chlamydial screening in those aged 25 and under who had access sexual health services or primary care.
"It is not a new approach and evidence shows it can be used successfully as a health care intervention," he said, adding it had immediate benefits to the individuals screened.
"Over 270,000 screens have been performed to date, and around one in 10 young people are testing positive.
"These results highlight the high burden of infection in people which would have been missed in the absence of a screening programme."