Campaigners have called on ministers to ban a vaccine which is routinely given to newborn babies.
They say there is growing concern that a mercury compound in the three-in-one DTP vaccine may cause autism and brain damage.
The vaccine, which protects against diphtheria, tetanus and pertusis or whooping cough, is given to infants of eight weeks and older.
However, the Department of Health has said there is no cause for concern and that the vaccine is safe for children.
Recent studies have found no link between DTP vaccines with thiomersal and autism and brain damage.
However, regulators in the United States and European Union have recommended that manufacturers should phase out the use of the compound wherever possible as a precaution.
The Department of Health said the current vaccine would be replaced as soon as a suitable alternative became available.
But a spokeswoman added: "All vaccines are tested for their safety and efficacy.
"Recent reviews by the Committee on the Safety of Medicines and the US Institute of Medicine found no evidence of any effect of low doses of thiomersal on childhood development."
On Tuesday, Prime Minister Tony Blair also dismissed claims saying there was no evidence to back up calls for the vaccine to be withdrawn.
But Bill Welsh, chairman of the pressure group Action Against Autism, urged ministers to ban the vaccine.
"We want an immediate end to vaccines containing mercury, particularly for children. This must stop today, this moment, immediately."
Speaking to BBC News Online, he said there were growing fears that the mercury compound in the vaccine could pose a threat to children's health.
He suggested that thiomersal in the DTP vaccine could cause mercury poisoning in some infants.
"We want every child who has been diagnosed with autism since 1990 to be properly examined to see if they have actually suffered mercury poisoning.
"The symptoms of mercury poisoning and autism are exactly the same."
Mr Welsh was speaking after he handed in a petition to the Scottish parliament calling for an inquiry into the safety of the vaccine.
He was joined by Dr Gordon Bell, a senior lecturer at Stirling University.
He said: "This issue has never been investigated properly. An inquiry is long overdue."