Tammy McLellan was on the point of having her youngest son, Alasdair, vaccinated with the controversial MMR vaccine when she suddenly questioned what she was doing. A year earlier another of her sons, Lewis, had been diagnosed with autism and she and her husband, Jim, fear the triple jab for measles, mumps and rubella may have triggered the life-long condition.

'I was going ahead with the vaccine for Alasdair when I found myself saying: 'I just cannot do this. What am I doing? If I have any doubt that MMR triggered autism in Lewis why am I doing this with Alasdair?'' she said.

The McLellans had listened to repeated government reassurances about the safety of the MMR vaccine but could not bring themselves to take what they believe is a risk with their youngest son's health.

Tammy said: 'The government talks about being pro-family and providing choice, but this situation feels like Big Brother. We think this is wrong, particularly for children who are so vulnerable and at such a young age. We feel giving the MMR vaccine is like Russian roulette -- 10 kids can have it and are fine, but the 11th child comes along and bang!'

Alasdair is now five years old but has still not been immunised against the three childhood diseases. The McLellans, who also have three older children, had decided to save up to pay for their son to receive single vaccines from a private GP, but the cost and threat of disciplinary action against doctors who offer parents an alternative made them question why the government is making it so difficult for parents to do what they believe is best for their child.

The McLellans believe so strongly that their son's right to safe immunisation has been infringed that they are seeking to take legal action, possibly using human rights legislation, to force the government to make single vaccines available. A lawyer has agreed to deal with Alasdair's case and is investigating the best way to take it forward.

Jim McLellan said: 'As far as we can see, making a legal case against the government is the only way forward. The government and health authorities are making it very difficult for anyone to get single vaccines and this is an indefensible position.

'There is no conclusive proof of whether this vaccine can trigger autism or not, but there appears to be some doubt put forward about how safe it is. Single vaccines are available in other European countries, and given that single vaccines were available in this country before the MMR was introduced and were considered to be safe, then I do not know why they should not be available now.

'I do not see why the government should force children to take the MMR.' Mr McLellan says that not only would the family need to pay for single vaccines but they could also jeopardise a doctor's career.

A handful of private doctors offer single vaccines for mumps, measles and rubella in the UK, but they have been put under intense pressure not to offer the alternative.

Lincolnshire GP Dr Mansfield is to be dragged before the General Medical Council, medicine's disciplinary body, by his local health authority for making the single vaccines available. He will appear before the GMC's Interim Orders Committee, charged with acting 'outside normal clinical practice'. The committee could ban him for 18 months, impose restrictions on his practice and refer the case for a full hearing of the council.

The threat of disciplinary action has prompted Dr Jean Knowles, the first NHS doctor in Scotland to prescribe single vaccines as alternatives to the controversial MMR triple vaccine, to stop giving single jabs.

In January all GPs in Scotland were warned that they face legal action if they give single jabs to children instead of the MMR vaccine. Scotland's deputy chief medical officer, Dr Andrew Fraser, wrote to family doctors telling them officially they must use the triple measles, mumps and rubella injection.

Mr McLellan said that 'the government is doing its best to prevent doctors from making single vaccines available. They are saying the MMR is the only way this vaccination can be done. If there is a question of doubt over its safety and they are denying us the alternative then it is infringing our right to immunisation.'

The lawyer who is acting for the McLellans, Fiona Moore of the firm Drummond Miller, said: 'Mr McLellan has taken legal advice on the options open to him to have the issue looked at in a judicial context.'

But legal experts point out that it will be a difficult case to take forward. Rosemarie McIlwhan, who is director of the Scottish Human Rights Centre, said the family may argue their case under article two of the European Convention on Human Rights, which is called the Right to Life.

She said: 'The family could argue that by not giving access to the single MMR vaccine, the state is not sufficiently protecting their right to life.

'If the family took this action the state would argue that there is not sufficient proof that providing single vaccines is any better for protecting life than providing the collective vaccine. It would come down to a judge deciding.'

Experts also point out that as this is not a life or death issue, the act may not be deemed applicable.

Bill Welsh, chairman of Action Against Autism, says it is unfair that a family has had to resort to legal action in order to have their child immunised in a way which they believe to be safe.

He said: 'This couple have witnessed with their own eyes one son's retreat into autism, and understandably they refuse to take a risk with the life of a second child.

'They wish to protect their younger son and have asked for and been refused single vaccines. If a child is not going to receive MMR he is better to have single vaccines than no vaccine at all. If the parents have made an informed decision in regard to their child's health, what right has the state to interfere?

'That a Scottish family has to take legal action in order to protect their child with a vaccine which is freely available worldwide brings shame on those in charge of child health in Scotland.

'This historic legal action is as a result of the lack of respect demonstrated by the department of health towards the genuine concerns of parents.'