More children will be separated by prison than divorce this Christmas, the Prison Reform Trust has said.
Of the 141 prisons in England and Wales, 85 are overcrowded
The organisation is calling on people to think about the distress faced by prisoners and their families.
It says putting criminals in overcrowded prisons and breaking up families does nothing to make the public safer.
This December the prison population in England and Wales will be nearly 81,000 - compared with 61,000 10 years ago.
In a message to mark Christmas and the New Year, the trust says it is estimated that every year over 150,000 children will see a parent locked up.
'Isolation and sadness'
Juliet Lyon, Director of the Prison Reform Trust, said while for some it can be a relief, for most children the imprisonment of a parent is "distressing and damaging".
"For many prisoners, being separated from their children will reinforce feelings of isolation and sadness," she said.
"Locking up so many people in overcrowded jails and breaking up so many families carries a huge price and does little to make us safer."
The trust says more than 132,000 people will have entered prison in England and Wales during 2007 and that 85 of the 141 prisons are classified as overcrowded.
Children of prisoners are three times as likely to suffer mental health problems compared with their peers and experience higher levels of social disadvantage, it says.
It says about 65% of prisoners re-offend within two years of release and for young men the re-offending rate is 75%.