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Such fillings or amalgam have been used for 150 years, but in recent years fears have been raised about mercury vapor being absorbed into the body.

Mercury poisoning can lead to memory loss and kidney failure, the Journal of the American Medical Association said.

But the report found no evidence of harm in the study of more than 1,000 children in the US and Portugal.  Read about Autism Mercury connection

US experts believe any lingering fears about the safety of such fillings should be dispelled by the report.

Silver fillings are made from silver, copper, zinc and other metals held together like glue by mercury.

Hopefully this will help reassure parents who are concerned about the safety of amalgam fillings in children   Dr Nigel Carter, of the British Dental Health Foundation

Originally, it was thought that a person's exposure to mercury was brief, lasting only when the filling was put in.

But in the 1980s scientists showed that amalgam continuously released vapour in the mouth, which is inhaled and absorbed by the body.

A Department of Health-commissioned review in the UK nine years ago found the use of dental amalgam was free from the risk of toxicity and, as a result, it is still used by NHS dentists, although white fillings are sometimes used for front teeth.

And despite other studies showing similar results, fears have persisted that amalgam fillings are not completely safe.

The researchers from the National Institute of Dental and Craniofacial Research, who conducted two studies in Portugal and the US, took a group of children with tooth decay but no existing fillings and gave them either silver fillings or white composite resin fillings.

After several years they were tested for changes to memory, co-ordination and nerve conduction.

Mercury

No deterioration was measured, although researchers did find those with sliver fillings had slightly higher levels of mercury in their urine.

Institute director Dr Lawrence Tabak said the levels of mercury were so low they were not causing a problem.

But he said: "It was clear that we needed to determine whether the potential risks of dental amalgam in any way outweigh its benefits and we needed to make the determination first in children, who may be more sensitive to any adverse effects of mercury.

"What's particularly impressive is the strength of the evidence. The studies evaluated mercury exposure in two large, geographically distinct groups of children and reached similar conclusions."

Dr Nigel Carter, chief executive of the British Dental Health Foundation, said: "These studies are the latest to find that there is no link between amalgam fillings and any medical disorder.

"Hopefully this will help reassure parents who are concerned about the safety of amalgam fillings in children.

"However, if parents are still concerned about their child having amalgam fillings, it is worth remembering that there are alternatives, such as white fillings, though your dentist will usually charge extra for these."