The split-up of the Home Office is happening too quickly and without adequate consultation, former lord chief justice Lord Woolf has warned.
Lord Woolf expressed concerns over individual liberty
The Department for Constitutional Affairs is due to become the Ministry of Justice and take over probation and prisons from the Home Office.
In a BBC interview, Lord Woolf said the reforms, set for 9 May, raised "concerns about our liberty".
But Lord Chancellor Lord Falconer said there would be "proper safeguards".
When he announced the changes at the end of March, Home Secretary John Reid said they were needed to meet the changing threats to the world.
After the split, the Home Office will concentrate on dealing with terrorism, security and immigration.
Lord Woolf told Radio 4's Today programme that, with added responsibility for prisons and probation, the lord chancellor could find it harder to safeguard the integrity of the court service and judiciary.
He said: "We should work it out beforehand and not wait until we have created the change and then somehow or other try to scramble to get it into place.
"This is a very big change for our constitution."
He added: "My concern in relation to this new Ministry of Justice is that it might, if it is absorbing what was previously the bulk of matters that the Home Office dealt with, be unable to have the sort of relationship we hitherto have had with the lord chancellor.
"There is close cooperation on matters where this is appropriate between the lord chancellor of the day and the judiciary.
"This arises from what is now history, but is still an important influence - the fact that the lord chancellor was head of the judiciary - so it is natural that the judiciary should listen to what he has to say and talk to him in confidence about their concerns.
"If the lord chancellor is watered down as to his traditional roles because of these new responsibilities he is being given, that would be worrying from this regard."
He said: "There has been no debate. Parliament has not considered this, but it is going to apparently happen on 9 May.
"I really think with our constitutional arrangements, we should be more careful about how these matters are dealt with.
"We have no written constitution which is entrenched and our constitution works through checks and balances and it is very important that if we are starting to alter the framework of checks and balances, that the matter is looked at carefully."
Lord Woolf also warned: "Our constitution protects our individual liberties - and these are not matters of concern of a financial nature, they are concerns about our liberty."
His words come a week after he said the plans to split the Home Office should be planned carefully to avoid repeating old problems in the new department.
But Lord Falconer told BBC Radio 4's Today programme: "This is do-able. We have got to understand the serious concerns raised and deal with them and that's what we are doing."
The plan would mean "better justice outcomes for the public", he added.
Lord Falconer said any delay to the Home Office split would "blight" the proposals, adding: "If we have a long period building up to a change, then what happens to the various organisations is they become paralysed leading up to the change."
He added: "These are things that have been carefully thought through. The issue about whether or not there should be a ministry of justice has been a public discussion over a long time."