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Thousands of children are having their tonsils removed unnecessarily, costing the NHS millions of pounds each year, according to experts.

They found seven in every eight children given tonsillectomies were unlikely to benefit from the operation.

NHS England has already said it plans to cut back on this surgery, along with other "ineffective" treatments, where the harms outweigh any gains.

Children may be given a tonsillectomy after several bouts of bad sore throat.

But Prof Tom Marshall and his team at the University of Birmingham found few who had the procedure fitted any of the main criteria:

more than seven documented sore throats in a year

more than five sore throats per year for two successive years

three sore throats per year for three successive years

Over-treated

They looked at the UK medical records of more than 1.6 million children between 2005 and 2016.

They found that out of more than 18,000 children who had had their tonsils removed during this time, only 2,144 (about 12%) had had enough sore throats to justify surgery.

Almost one in 10 had suffered just one sore throat before being offered the surgery, they report in The British Journal of General Practice.

Two or three children per 1,000 underwent tonsillectomy annually between 2005 and 2016 - but fewer than one in eight of them met the recommended criteria.

Based on these findings, the researchers estimate that 32,500 of the 37,000 child tonsillectomies in 2016-2017 in the UK were unnecessary, costing the NHS £36.9m.

Prof Marshall said that while a tonsillectomy might be justified for those most severely affected, "research suggests children with fewer sore throats don't benefit enough to justify surgery, because the sore throats tend to go away anyway".

Although complications from the surgery are rare, they can be severe.

NHS England said: "This study shows why the NHS, the medical profession and patients are working together to phase out operations that don't work or are inappropriate, which means better patient care and a more efficient use of taxpayer funding."

ENT UK, which represents surgeons, said the profession carefully considered which children should be offered a tonsillectomy, as per guidelines, but using clinical judgement.

A spokeswoman said another common reason for offering tonsillectomy is a condition called obstructive sleep apnoea, where enlarged tonsils block the airways.

"There is also evidence from the literature that following the introduction of evidence based guidelines and a reduction in tonsillectomy surgery from 2011, there is an increasing rate of admissions of patients in the UK with complications of acute tonsillitis, including peritonsillar abscess and deep neck space infections which have a significant morbidity, small but definite mortality, and a significant cost to the health economy."