Ex-Home Secretary Charles Clarke has criticised proposals to split up his former department, following a series of controversies.
Mr Clarke lost his job in last May's Cabinet reshuffle
Mr Clarke, who was sacked last May over a failure to deport foreign criminals, told Sky News he disagreed with his successor, John Reid.
"I think he's wrong," he said, adding that it could make the problems worse.
Mr Reid wants to split the Home Office into two departments, dealing with security issues and justice.
Mr Reid said parts of the Home Office were "not fit for purpose", when he took over last summer.
It has continued to hit the headlines with stories ranging from a lack of prison places to Britons who had committed crimes abroad but whose case files were not entered on the police computer.
In an interview on Sunday, Mr Clarke said that Mr Reid's recent decisions to write to judges to remind them of sentencing guidelines, and to suggest splitting up the Home Office, were both mistakes.
"I think the problem with the department is a lack of co-ordination between its various elements," he said.
"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".
Another former Labour home secretary, David Blunket, has said the split would not be good for balanced government as it would leave only two powerful figures in Cabinet - the prime minister and the chancellor.
But Jack Straw, who was Labour home secretary from 1997 to 2001, has said he had not backed such a plan then, but terrorism had since become an "overwhelming" issue and he now thought it was a good idea.
And Mr Reid has said: "We have to look at all of the options that are available to us with a view to protecting the nation, protecting the public, ensuring national security and putting the public interests first in everything we do."
Since he took over he has been reviewing the way the Home Office works and its structure, but says it will take two and a half years to fix.
Under Mr Reid's recommendations the security department would be responsible for anti-terrorism policy, immigration and the security services.
The Ministry of Justice would have control of probation, prisons and stopping re-offending.
The Conservatives have called the plan an "admission of failure" and say breaking up the Home Office will "compound existing problems" and create new ones.