Childhood obesity in Britain has reached epidemic levels and is likely to become even more common as children become less active, a leading health expert warned today.

Dr John Reilly, from the University of Glasgow, told a conference in Bristol that the growing number of obese children was likely to have serious implication for the general health and psychological welfare of the population.

At a joint conference of the Association of the Study of Obesity and the University of Bristol, Dr Reilly said obese children were now much more likely to remain overweight as they went through adulthood than they were 20 years ago.

He warned that they were at an increased risk of developing heart disease, diabetes or problems with their joints and bones in later life.

"It is a fairly common perception among families and health professionals that it [childhood obesity] does not matter that much," he said.

"But childhood obesity does matter. It has adverse health implications both in the short term and the long term.

"We will get a lot more long standing obesity than we have ever had previously - and that is a lot more dangerous."

After his presentation, Dr Reilly said childhood obesity was a problem that had been around for two decades but which had only been widely recognised in the last few years.

"The epidemic started in the mid-1980s and as far as we can tell it is becoming more and more common," he said.

"We have seen very dramatic increases right through the 1990s, and all the signs suggest it is going to get worse before it gets better."

Dr Reilly said obesity among the young was becoming more common because children were leading much less active lifestyles - watching more television and playing video games - while being taken to school by car rather than walking.

"Both parents and children are much less active than they were," he said. "The environment now promotes obesity in a way that it did not do before."

Dr Reilly said obesity in the young had short-term health implications, such as hypertension, high cholesterol and respiratory problems.

But he warned that these were unlikely to seriously affect children until they became much older, describing obesity as a "silent problem", the effects of which only became apparent during adulthood.

He said childhood obesity was also linked to a number of mental health problems such as low self-esteem, caused by the teasing and bullying overweight children were often forced to endure.

"Obese children are subject to a lot of psychological pressure," he said.

Dr Reilly also warned that obese children have poorer social, educational and economic prospects in adulthood than thinner young people. "Adults who were obese as children also have poorer social, educational and economic prospects," he said.

"Childhood obesity has a high cost in health and economic terms and we should be making greater efforts to prevent it."

Tomorrow the conference is due to hear the findings of a long-term study of children's diets.

Dr Pauline Emmett, one of the researchers on the Avon longitudinal study of parents and children, said: "We found that children's diets are influenced by the educational level of their mother and that groups with the least educated mothers had higher levels of obesity.

"Our findings show that it is important that health professionals have a role especially in encouraging the less educated mothers to follow best practice."