Britain - where medics still inject mercury into babies

But Britain is now understood to be the only developed country in the world to take this view. From Japan to the United States, mercury has been withdrawn from the child vaccination schedule precisely on the ground that doubts exist about its safety.

The debate is not about whether the vaccines should be administered: mercury-free jabs are available on the NHS - and, in Scotland, to anyone who demands them. Nor is it a scientific argument restricted to the domain of experts: parents’ groups consistently argue that it does not require a PhD in chemistry to establish that mercury should not be injected into the body of a eight-week-old baby.

It is instead an argument about whether parents should be told what is in the vaccine - and whether they should be given the power to decide which vaccination is best for their child.

The NHS uses a DTwP vaccine bought from a company called Aventis. Unless parents say otherwise, this is the vaccine which will be issued. It protects against diphtheria, tetanus and whole-cell pertussis (whooping cough). The wP means a whole dose of dead pertussis toxin, in a small enough dose for the infant’s immune system to learn about the disease, and how to protect against it.

"Mercury is a proven toxin so strong that the contents of a thermometer could pollute a small lake"

As a preservative, DTwP uses thimerosal - sometimes spelt thiomersal - which is 49.6 per cent ethyl mercury. Each injection contains 25 micrograms of mercury - which means 75 micrograms over the three-jab course in the first 16 weeks of the baby’s life.

The mercury-free alternative, also ordered by the NHS, is called Infanrix, produced by GlaxoSmithKline. The diphtheria and tetanus components are the same - the difference is that it does not use whole-cell pertussis. This is a crucial distinction.

In the 1970s, research was published suggesting that the whole-cell pertussis was linked to neurological disorders. This led to a worldwide scare and parents left infants unvaccinated, rather than risk the jab.

Medical authorities the world over had outbreaks of whooping cough on their hands. The decision was taken to produce a new type of vaccine which allayed parents’ concerns.

Scientists’ response was to use only the components of the pertussis vaccine which were absolutely necessary to provide immunisation. The result was acellular pertussis - DTaP.

Infanrix is part of the new generation of DTP jabs. Its components - pertussis toxin, filamentous haemagglutinins and pertactin - are purified and detoxified then included in the jab.

This process is naturally more expensive than just including whole cells of the pertussis bacillus. So Infanrix costs about twice as much as DTwP - which has made it prohibitively expensive to the third world vaccination programmes carried out by the World Health Organisation.

Infanrix is by no means the only DTaP on the market. There are scores of products made by various manufacturers - each competing with the others on effectiveness ratings and how few side effects are produced.

There is far less competition making DTwP jab - which is regarded in the richest countries as old technology whose time has passed. Also, there is no medical need to use mercury as a preservative when the new DTaP vaccines have none.

This is perhaps why the DTwP does not have a brand name. It is not, by and large, something any parent would request over DTaP - even though statistics comparing the two are hard to extract from the UK health service.

All vaccines are tested against each other, in studies normally involving tens of thousands of children. In each test which has ever been performed, Infanrix - and every DTaP vaccine - has been given a far superior "safety profile" than DTwP.

This has nothing to do with mercury. The problem is the so-called "junk cells" which exist in the whole-cell pertussis vaccine, but have been taken out of the DTaP jab. They are found to trigger a range of unpleasant and unnecessary side effects in infants.

"This is an argument about whether a child’s parents should be told what is in the vaccine"

Even in the hotly-contested world of vaccine wars, it is now beyond dispute that infants injected with DTwP are far more likely to suffer fever, convulsions or show protracted crying that could last for up to two days. All of these are triggered by the "junk cells".

Data filed in the Karolinska Institute in Stockholm shows that the figures vary - some say that babies are three times as likely to suffer convulsions after the DTwP jab, some say 1.8 times more likely. But none dispute the trouble caused by junk cells.

Britain’s defence - behind the use of mercury, as well as junk cells - is that the DTwP vaccine works better against whooping cough. The Department of Health argues that a child given DTaP is "twice as likely" to contract whooping cough.

But no studies quoted by the government directly compare the two vaccines which Britain is actually using. In any case, one study - conducted a decade ago - showed the likelihood of whooping cough was one in a thousand with DTwP and two in a thousand with DTaP. These are the odds.

David Geier, one of the scientists who produced a recent report suggesting a link between thimerosal and autism, said that debate has moved on since 1994 and the UK no longer needs reports.

"Look at the United States. Look at Japan. They have both been using DTaP for years - and do we see any outbreak of whooping cough there? It now doesn’t matter what a few studies found - we can look at entire countries."