Charles Clarke has condemned home secretary John Reid's plans to split the Home Office as "wrong", saying the move would only make problems at the department worse.
The Norwich MP, who was sacked as home secretary over the foreign prisoners scandal last May, said the main problem with the Home Office was the lack of coordination between the different branches - which would not be solved by a split.
"I think he's wrong," Mr Clarke said of his successor. "I think the problem with the department is a lack of co-ordination between its various elements.
"Between immigration and asylum on the one hand, police, prisons and probation, there simply isn't enough co-ordination between them.I think dividing the Home Office would make those problems far worse."
Within weeks of taking over the Home Office, Mr Reid declared it "not fit for purpose" and prompted a full-scale review of its processes. This has now been presented to Tony Blair and is expected to recommend the department be split in half.
The plans are thought to suggest that a security department be created to deal with counter-terrorism policy, immigration and the security services, while a ministry of justice would be responsible for probations, prisons and reoffending.
The Liberal Democrats have backed the idea but although the Conservatives have called for a separate cabinet minister for homeland security, they warned the split could create more problems.
Yesterday Mr Clarke argued that the foreign prisoners scandal - where 1,000 foreign nationals were let out of jail without being considered for deportation - was a problem of information sharing, and would not be helped by splitting the Home Office.
He added: "I think a number of the things that have happened recently have been caused by decisions that John has taken - for example to decide to want to split the Home Office and deal with it and then to write the letter to the judges [about sentencing] that he did.
"They were both acts which took the issue forward in ways that weren't necessary."