The split of the Home Office needs to be planned carefully to avoid repeating old problems in the new department, a former lord chief justice says.
Prisons will be under the control of the new Ministry of Justice
The Department for Constitutional Affairs will become the Ministry of Justice and take control of probation, prisons and prevention of re-offending.
But Lord Woolf said the DCA "already had plenty of headaches" itself.
He said if it reproduced the recent Home Office problems "then it's not going to achieve any positive purpose".
Criticism of the increasing prison population and sentencing have been levelled at the Home Office recently.
Under the split, the Home Office will concentrate on dealing with terrorism, security and immigration.
Lord Woolf said: "I hope that the planning of these changes is going to be very, very carefully worked out, because they aren't straightforward.
"There will be very substantial managerial consequences to deal with."
Lord Woolf, appearing before the home affairs select committee, raised concerns about resources under the new structure.
"For many, many years the civil justice side has been the poor relation.
"Resources have constantly gone from civil justice into the criminal justice system because of the demands, particularly of criminal legal aid and complex prosecutions."
Lord Woolf, who retired in 2005, added: "What I would be concerned about if I were still chief justice is that the prisons have an amazing capacity to absorb money as the population increases more than expected.
"I am worried that as far as I know no steps are being taken to ring fence the resources which are available for the justice system as a whole, and to the courts in particular.
"Care needs to be given to that," he warned the MPs.
When he announced the changes at the end of March, Home Secretary John Reid said the changes were needed to meet the changing threats to the world.