There is no evidence that a mercury- based preservative used in vaccines is linked to children's behavioural problems, researchers have said.
Some concern over the safety of the jab has been expressed
A study by Bristol University's Children of the 90s project backs up the government's case that thiomersal is not dangerous to children.
Thiomersal is used in the jab against diphtheria, tetanus, whooping cough and Hib, which is being replaced.
The government says there are better vaccines with fewer side effects.
After reviewing the immunisation records of almost 13,000 children, researchers said they had found no evidence linking the preservative to developmental disorders in youngsters.
They said the figures appeared to show that the earlier the child was immunised with the vaccine, the fewer the problems reported as they got older.
Even after taking into account factors such as birthweight and whether the child was breast-fed, the effect appeared to be beneficial.
A spokesperson said: "In the face of this evidence, we would support the view that the dangers posed by contaminated vaccine vials far outweigh any potential risk posed by thiomersal."
The team looked at 23 different measures of the child's behaviour and development, such as hyperactivity, difficulties with speech and special educational needs.
The new thiomersal-free five-in-one jab is being introduced in the UK this month.