Some frequently prescribed antibiotics could be rendered "ineffective", researchers have warned after finding high rates of resistance to the drugs among children with a common condition.

Antibiotic resistance among children with urinary tract infections (UTIs) caused by the infectious bacteria Escherichia coli (E.coli) is high, the study concludes.

Researchers from the University of Bristol and Imperial College London reviewed the results from 58 studies from around the world involving almost 78,000 E.coli samples taken from patients.

Their study, published in the BMJ, showed a high global prevalence resistance to some of the most commonly-prescribed antibiotics.

The researchers found that in OECD countries, more than half of all samples were resistant to ampicillin and almost a quarter were resistant to trimethoprim.

Three in 10 samples were resistant to co-trimoxazole and 8.2% were resistant to co-amoxiclav.

The researchers said resistance was substantially greater in non-OECD countries, where almost four in five samples were resistant to ampicillin, almost 70% were resistant to co-trimoxazole and three in five did not respond to the antibiotic co-amoxiclav.

"Our findings detail global high-level resistance to some of the most commonly-prescribed antibiotics for children in primary care, which could result in several drugs becoming ineffective first-line treatments in many countries," the study concludes.

E.coli is responsible for over 80% of all urinary tract infections in children.

NHS Choices states UTIs are "a relatively common infection during childhood". It is estimated that around one in 10 girls and one in 30 boys will have had a UTI by the time they turn 16.

"Prevalence of resistance to commonly-prescribed antibiotics in primary care in children with urinary tract infections caused by E.coli is high, particularly in countries outside the OECD, where one possible explanation is the availability of antibiotics over the counter," said lead author Ashley Bryce, a PhD student at the Centre for Academic Primary Care at the University of Bristol.

Study co-lead Dr Ceire Costelloe, from the Health Protection Research Unit in Healthcare Associated Infections and Antimicrobial Resistance at Imperial College London, added: "The results also suggest previous antibiotic use increased the subsequent risk of E.coli resistance to that particular antibiotic - for up to six months after treatment."