Save the Children UK says that without special help their education may suffer - but that teachers have had no specific guidance on how to support them.

"At any given time, thousands of schoolchildren in the UK have a parent or carer in prison," said the charity's UK programme director, Judy Lister.

"Effective support now will mean that some of our most disadvantaged children have the chance to make the most of their education and live up to their potential."

"I couldn't tell mum how I felt because she already had enough to cope with" - one respondent

So after consulting various organisations working with prisoners' families the charity has produced an information pack to help teachers support inmates' children.

Save the Children spokeswoman, Joanne Bailey, said the figure of 125,000 was a conservative estimate, based on data produced originally by the Cambridge Criminology Institute. She said that if jailed step-parents and siblings were included, some observers calculated that as many as half a million children around the UK might be affected.

The charity's initiative came from its experience of working with the children of women in Holloway Prison in London.

She said: "A pattern was emerging that they didn't have enough support in schools."

'Best intentions'

One teenage girl whose mother was jailed, and who felt the need to confide in someone, talked to her teacher. The teacher, with the best of intentions, told the girl's classmates without her knowledge.

"This mortified her and only added to her embarrassment, guilt and shame," Ms Bailey said. "The teacher didn't think about the consequences of teasing and bullying. Children can be very cruel. It was totally the wrong thing to do."

The information pack for teachers warns them not to ask about the parent's crime, and tells them to be "non-judgmental" of the child, as he or she has done nothing wrong. But they should avoid treating the child as a victim or being over-protective.

Ms Bailey said it had come across clearly during Save the Children's research that teachers thought it was an issue but they did not know how to deal with it.

Growing problem

"Day in and day out they are dealing with children in this situation and they are muddling through. We are not trying to add to their workload but to help them out."

Save the Children says that apart from the social stigma, pupils may also miss lessons to visit their parent in prison, or be forced to move house because their primary carer is in jail. They may also have to take on extra responsibility to care for younger brothers and sisters.

With a rising prison population, it was clear that the problem was not going to diminish.