Vicki Anderson will never forget the moment she heard her son Joseph shrieking in the bathroom. "When I ran in, he was screaming in the corner of the bath," she remembers. "I pulled him out, unzipped his Babygro, and his skin came off in my hands."

Anderson had been running the bath for her children, then two and three years old. She put the hot water in - not a lot - then was interrupted by the children shouting in their bedroom. Two-year-old Joseph was out of his crib. "He was hiding and messing around," she says. "I turned to say something to Francesca and no more than a minute later I heard a scream. I knew instantly what had happened."

Joseph was airlifted to Selly Oak hospital in Birmingham, where doctors gave him a 7% chance of survival. He was in intensive care for three and a half months, and suffered burns over 76% of his body. He had dropped his toy dinosaur into the hot bath and, while trying to retrieve it, had fallen into the water. Now a cheery 12-year-old, he has spent his entire childhood recovering from the split second when he tumbled into the bath. "I've lost count of how many operations he has had," says Anderson. "He was in theatre twice a week for the first few months after the accident, and in hospital for a year. He was known as the 'blue baby' because he would stop breathing so often. The worst bit of all was when he fell into a coma two-and-a-half months after the accident. We all had to say goodbye to him."

Although Joseph's injuries are extreme, this is a frighteningly common story. Every day in Britain, a child under five is admitted to hospital with serious burns from bath water. "People don't realise that falling into hot water is exactly the same as falling into a fire," says Labour MP Mary Creagh, who is scheduled to take a delegation of experts from her Hot Water Burns Like Fire campaign to meet the prime minister this week. Creagh hopes to persuade Gordon Brown to back legislation that would require all new buildings in England and Wales to be fitted with thermostatic mixing valves - devices that prevent water coming out of bath taps from reaching a temperature that will burn skin (this legislation is already in place in Scotland). Such measures will not eradicate all injuries, but burns specialists and charities such as the Child Accident Prevention Trust and the Royal Society for the Prevention of Accidents believe that the legislation would save enormous suffering.

"Right now, it is just so easy for these burns to happen," says Creagh, herself the mother of a four-year-old boy. "You'll be running a bath and nip to answer the phone or grab a towel. The first thing you hear is a terrible scream. Parents tell me they rushed into the bathroom and saw what they thought was tissue paper in the water. Then they realised it was their child's skin." Elderly people are also at heightened risk from bath-water burns - their skin is thinner, and they may be unable to haul themselves out when they feel the pain. Of the 600 people a year who suffer severe bath-water scalds, three-quarters are children (mainly under-fives). Most of the others are elderly (15 pensioners a year die from hot-water burns). Princess Margaret burned her feet in the bath three years before she died - an injury from which she reportedly never recovered. People under the influence or alcohol or drugs are also more at risk, as are those with disabilities.

Most of us underestimate wildly what hot water can do. At 60C, water can burn skin in about two seconds. At 70C it takes less than a second. Studies show that, in the average home, hot water comes out of the tap at more than 60C - in some cases, it is as hot as 80C.

Actor Amanda Redman, who fronts Creagh's campaign, suffered burns as a child when she pulled a pan of soup over herself; she is badly scarred on her left arm.

"These injuries can be truly horrendous," says Dr Keith Judkins, a consultant at the Pinderfields Burns Centre in Wakefield. "When hot water burns the skin, it damages - and sometimes kills - the superficial layers. The underlying layers become inflamed, and fluid is lost into the surrounding tissues. The body can then go into shock if proper treatment isn't started promptly."

For weeks after the injury there will be painful daily wound dressings. "There may have to be many skin grafts - where thin layers of skin are taken off other parts of the body, and placed over the wound. It is a painful and distressing process for both the patient and their family, and there is always a risk of infection," says Judkins. "That can be life-threatening." Over the next two to three years, after the wound has healed, the child will have to wear pressure garments to minimise scarring, and will probably need reconstructive surgery. As they continue to grow - until they are 18 or 20 years old - many will have to undergo repeated skin grafts. The result is "scarring, disfigurement - and sometimes disability, if the burned tissue is over a joint".

Of course, these physical wounds are only part of the story. "Bath-water scalds are called life-changing injuries by surgeons," says Katrina Phillips, chief executive of the Child Accident Prevention Trust. "Surgeons know that children will often be left scarred for life both physically and mentally by these injuries." Anderson remembers how once, when Joseph was about six, he became very distressed when having his dressings changed. "He was shouting 'I'm wrinkly all over! I belong on another planet!'" Many burns units, including Pinderfields, run "burns clubs" and holiday camps where children can be with others in the same situation - children who will not tease them because of their scars. Many victims - and their families - need psychiatric help to recover.

"We call these 'whole-family injuries'," says Judkins. "They permanently change the way a family operates. There are all the visits to clinics, and the surgeries and a huge psychological impact." "It can," adds Phillips, "also mean a life sentence of guilt for the parents."

Creagh sees this first hand in the letters she gets. One father from Somerset recently wrote to tell her about his one-year-old daughter, who had been "mucking about" with her four-year-old sister next to a hot bath. "They were only left alone for a minute, but the baby clambered in," says Creagh. "She survived the initial burns but had several heart attacks in hospital and eventually died from an infection. The agony for that family is indescribable."

At around £80, a thermostatic valve kit certainly seems cheap at the price. As for Anderson, "I think of myself as the luckiest mother on the planet," she says. "I've still got my baby."

How to avoid hot-water burns

· Always run the cold water first, then add in hot to get it to the right temperature - never the other way round.

· Test the water before you put your child in the bath; if it feels at all hot to you, it's too hot for a baby or toddler. Use your elbow - if the water feels pleasantly warm after about a minute, it's OK.

· Never leave children unsupervised in the bathroom near a hot bath - even for a moment to get a towel or answer the phone.

· Beware when adding more hot water to a cooling bath.

· Install a thermostatic mixing valve in your hot-water system (it will cost around £80).

Contact the Thermostatic Mixing Valve Manufacturers' Association (safehotwater.co.uk). Campaign info: hotwaterburnslikefire.org.uk