Mohammed Hanif and his wife lost their first baby to a disorder so rare that if affects only a handful of children in the UK. It was too rare for doctors to understand what had happened to the child until his autopsy.

"For three days we were on top of the world. We had a beautiful son - the most gorgeous thing you could see," said Mr Hanif.

"Three days later he died in my arms."

The baby had an enzyme deficiency that makes it hard for the body to break down proteins. If the proteins are not processed, they can produce toxic ammonia, which can swiftly cause brain damage and death.

But the condition can be treated and stabilised. The Hanifs had two more children, both with the same disorder.

But both are able to lead normal lives, apart from care with their diet, because they are on daily doses of the chemical carbamyl glutamate.

"My son will be 10 in July," said Mr Hanif. His daughter is nearly eight.

"They go to normal school. They fit in. Looking at them, you would not think they had anything wrong with them.

"Until about four weeks ago, I was trying to block it out and just get on with life."

Then suddenly, when his wife went to renew the children's prescription at the pharmacy at South Tees hospital where the raw chemical is turned into palatable tablets, she was told supplies of carbamyl glutamate had run out.

The pharmacist said she was sorry, but the children would have to take daily doses of two other compounds, sodium benzoate and sodium phenylbutirate, in liquid form.

Once, in a crisis, his son had been given these drugs intravenously, and they had caused him to vomit.

Mr Hanif was not at all happy with the new prescription - and he was angry that he had not been told earlier by his consultant that there were supply problems.

The Hanifs tried to give their children the new medicines, but the children would not swallow more than the odd spoonful - they should have had 24 each day - not only be cause of the taste but because it made them feel ill.

"It made them feel sick. They had tummy pains. They would lie down and moan," said Mr Hanif.

"By the weekend my son was vomiting. On Tuesday night, after four to five days, we took him to hospital but the doctor said he didn't know what to do.

"On the Thursday morning, my son was really ill. He was hallucinating really badly. The following day, my daughter, who has got a milder version, was admitted to hospital too."

Mr Hanif contacted the Middlesbrough MP, Stuart Bell.

He said: "The children were on drip feeds and being sick. They were clearly not very well and the family was desperate.

"I got involved by writing to the health secretary, Alan Milburn, and to Lord Hunt (responsible for medicines)," he said.

Eventually temporary supplies of the chemical were secured from another hospital.

But the chemical manufacturer it used to buy from will no longer sell it carbamyl glutamate.

For the long term, South Tees has decided it has no alternative but to pay the high cost of the licensed drug - £80,000 for each child - which will be supplied solely by Orphan Europe.