The government has announced plans for a new combined jab for babies.
People are concerned over the safety of the new jab
The vaccination will protect children against diphtheria, tetanus, whooping cough, Hib and, for the first time, polio in a single shot.
Health campaigners have expressed concern about the safety of a new combined vaccine amid fears it could overload babies' immune systems.
But health officials say the new vaccine is safer even than the four-in-one jab it will replace.
Unlike the vaccine currently in use, the new version will not contain mercury as a preservative.
A US study has suggested that whooping cough vaccine containing the metal is linked to autism.
However, the Department of Health says there is no evidence of such a link, and that its removal is nothing to do with safety issues.
Despite this, the move has been universally welcomed as being "long overdue".
The new vaccine is being introduced to remove the tiny risk that children could contract polio paralysis from the current oral vaccine, which contains a live sample of the virus that causes the disease.
The new vaccine will contain an inactivated form of the virus. The switch is now possible because a global campaign to eradicate polio means the risk to UK children is now extremely low.
Overall, the new five-in-one jab will contain fewer "live" elements than its four-in-one predecessor
Jackie Fletcher is the founder of Jabs (Justice, Awareness and Basic Support), a support network for parents who believe their children have been damaged by vaccines.
She has raised concerns about the safety of giving babies jabs containing multiple vaccines.
She said the Department of Health had to recall thousands of children two years ago for booster shots of Hib meningitis after concerns about the effectiveness of the current four-in-one jab.
Ms Fletcher said: "With five-in-one vaccines we would want to know what safety trials have taken place.
"How did they find out it was safe to do it in this combination?
"Increasing the combinations increases the potential for an adverse reaction and restricts choice for parents, when the government said it wanted to improve choice."
Dr David Salisbury, an immunisation specialist with the Department of Health, said the new vaccine had been formulated in consultation with parents and health professionals.
It had already been used in Canada for seven years, where follow-up studies had revealed no problem with side effects.
Dr Salisbury also stressed that there was no evidence of a link between mercury and autism.
"There is extensive research from this country and a huge review from the US, and both conclude very clearly that mercury does not cause autism when it is present in vaccines.
"Please do not delay having your child vaccinated. Our vaccines are extremely safe.
"If you delay and you do not protect your baby against, particularly whooping cough, at a young age, then you put your baby at very high risk."
Dr Salisbury also said the government was looking "very carefully" at the feasibility of introducing a separate, new vaccine for bacterial meningitis.
Liberal Democrat health spokesman Paul Burstow hailed the removal of mercury form vaccines, but said the science in the 5-in-1 jab had to be "sound".
"Parents and the public must have confidence that all possible risks have been discounted before the new combined vaccine is put into use," he said.
Shadow health secretary Andrew Lansley said: "The government haven't learned the lessons of the MMR controversy.
"It is not just that the vaccination programme must be safe and effective, we have to carry public confidence and that means better public consultation and
independent appraisal of new vaccine arrangements."
The new vaccine will be given to babies over two months old from September.