Women should be tested for chlamydia every time they have a new sexual partner because screening alone is unlikely to cut their chances of pelvic disease, researchers said today.

Chlamydia is the most common sexually transmitted infection in the UK and can cause pelvic inflammatory disease, leaving women infertile.

Now researchers are warning that women should be tested every time they start seeing a new partner, even if it is just a few months since they were last tested.

They are worried women may see themselves as having the "all-clear" following a negative result when they are actually still at risk.

Their research found it was unlikely that one-off screening for chlamydia will prevent women developing pelvic inflammatory disease in the following year.

Experts from London hospitals conducted a trial on 2,529 sexually active female students from 20 London universities and colleges, with an average age of 21.

Their samples were either tested straight away or stored and tested 12 months later. Of women tested straight away, those found to have chlamydia were treated.

All results were then checked against the number of women developing pelvic inflammatory disease over the next 12 months.

A total of 68 (5.4%) out of 1,254 women who were screened immediately had chlamydia and 75 (5.9%) out of the 1,265 screened after a year also had it.

Fifteen (1.3%) of the immediately screened women went on to develop pelvic inflammatory disease compared with 23 (1.9%) of the women tested after a year. The results suggested an 80% reduction in the risk of pelvic inflammatory disease in women treated for chlamydia infection, although the number with the disease was small.

The authors said most cases of the disease - 79% - were in women who tested negative for chlamydia at the start of the study. This suggests women picked up the infection at some point in the next 12 months.

Writing online in the British Medical Journal (BMJ), they said: "Although some evidence suggests that screening for chlamydia reduces rates of pelvic inflammatory disease, especially in women with chlamydial infection at baseline, the effectiveness of a single chlamydia test in preventing pelvic inflammatory disease over 12 months may have been overestimated."

The research could call into question the effectiveness of the Government's chlamydia screening programme which is targeting under-25s, who have particularly high rates of the infection.

The researchers said: "Although some evidence shows that screening reduced rates of pelvic inflammatory disease, especially in women with chlamydial infection at baseline, the absolute number of cases prevented was small.

"Our findings suggest that to prevent one case of clinical pelvic inflammatory disease over 12 months, it may be necessary to screen 147 women for chlamydial infection or to treat 13 women who are positive for chlamydia.

"If the incidence of pelvic inflammatory disease in women with chlamydial infection has been overestimated, and particularly if it is less than 10%, then the cost effectiveness of screening might be exaggerated."

The authors said women who are high-risk should be checked more than once a year.

"Policymakers might consider focusing on more frequent testing of those at higher risk, such as women with a new sexual partner or a recent history of chlamydial infection."

According to Health Protection Agency figures on people attending GUM (sexual health) clinics, there were 123,018 cases of chlamydia in 2008 among men and women of all ages, up from 121,791 in 2007 and 113,713 in 2006.

Of the cases in 2008, 61,155 were among women.

In an accompanying BMJ editorial, Jessica Sheringham, from University College London, said: "The justification for investing in chlamydia screening rests primarily on the assumption that pelvic inflammatory disease is common enough to constitute a public health problem, and that chlamydia is a major cause of the condition.

"However, estimates of its incidence vary greatly; in an audit of records from just one clinical centre, rates of diagnosis ranged from 0% to 5.7%."

Nevertheless, she said screening was often accompanied by sexual health advice and, after a positive result, treatment of the current sexual partner.

This could lead to prevention of other sexually transmitted infections if people take fewer risks, she said.

A spokeswoman for the Department of Health said: "The aim of the National Chlamydia Screening Programme is to detect undiagnosed chlamydia through proactively offering testing to young people aged under 25 in a variety of health and community settings.

"The programme offers regular chlamydia screening to young people so that they are tested annually or when they change partner.

"We are working to implement the recommendations of the Public Accounts Committee to improve value for money and effectiveness of the programme."

In January, a report from MPs said the Government had no idea if the chlamydia screening programme was having an effect.

Inadequate measures were in place to check progress of the screening programme across health trusts in England, they said.

The study, from the Public Accounts Committee, came after a damning National Audit Office study in November said millions of pounds of taxpayers' money had been wasted on the strategy.

The PAC report said that five years after the programme's launch, just 4.9% of people aged 15 to 25 were being tested despite the target being 15%.

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% in 2008/09 against a 17% target.

More than half of all new cases of chlamydia in 2008 were among under-25s.

Dr Pippa Oakeshott, reader in general practice at St George's, University of London, who led the research, said women with a new partner should get tested within a few days of unprotected sex.

"Given that it takes a little while to get round to having a test, I don't think it could be too soon."

She added: "Chlamydia screening does work but it needs to be easier for people to get tested, such as in colleges, in clubs or via online."