But critics said that the proposal could lead to more youngsters committing suicide in prison and teenage inmates being turned into career criminals by mixing with older prisoners.
Announcing the plan yesterday the Home Office minister Paul Boateng said that society regarded people over the age of 18 as being adults.
He released a government consultation paper that stated: "The present legislation provides an artificial separation at the age of 21, which does not mark the child/adult transition, and which cannot reflect the maturity or vulnerability of individuals."
Mr Boateng said that young adult prisoners would not be at any greater risk in adult prisons than they had been in young offenders' institutions(YOIs).
"You can meet 18 and 19-year-olds who are more hardened and a greater threat to other 18 and 19-year-olds than some prisoners in their mid to late 20s, in terms of bullying, intimidation and the like."
Frances Crook, the director of the Howard League, said that nine young prisoners aged between 17 and 20 had already committed suicide this year in YOIs.
"If you put these young people in prison with adults, the amount of violence and self-harm will increase. Many of them will be terrified. Others will see it as a badge of honour to be treated as an adult prisoner. Neither of these attitudes I would welcome."
Ms Crook said the plan was a "cost-cutting exercise" that followed the Home Office decision to set up a new youth justice system for offenders under 18, which will result in 30 youth jails being set up for nearly 3,000 young offenders.
The director-general of the Prison Service, Martin Narey, said the plan to move offenders aged 18 to 21 into adult jails would mean that such prisoners could be treated with "greater flexibility". Some prisoners under the age of 25 would exhibit problems common to other young inmates and would need specialist treatment, while some would not.
Mr Narey said he hoped that in time a series of young-adult prisons could be set up across England and Wales, each with a specialist staff trained in working with young people.
He said: "Clearly I would not want to put any vulnerable 18-year-old in a situation where they are exposed to dangerous adults."
The first of the new "young- adult" prisons was likely to be set up at the notorious Feltham YOI, west of London, which would be split into two establishments, one catering for children under 18 and the other for prisoners in their late teens and early 20s.
But Prison Service sources admitted that plans for other "young-adult" prisons were long-term and that the 6,500 sentenced prisoners aged 18 to 21 would initially be moved into existing adult jails, where they may be accommodated in separate wings or units.Reuse content