Home Secretary John Reid has said he is "not going to quit"- but expects to find more problems at the Home Office.
Mr Reid said reforming the Home Office is not "a mission impossible"
Reforming the department would take two and a half years, he told the BBC, adding that he had the "endurance" and "determination" to "see it through".
He said the public were not interested in ministers being changed - they wanted the Home Office to be changed.
"The fact is I'm in there changing things and I will continue to change them," Mr Reid told Radio 4's Today.
"There's one thing that's certain, I'm not going to quit," he added.
Mr Reid's comments about the job taking two and a half years suggests he expects to remain in his post as home secretary, rather than challenge Gordon Brown to succeed Tony Blair when he steps down this year.
Asked about that, he said he was working "very closely" with Mr Brown and "as late as last night I was discussing these matters with Gordon".
Asked directly if that meant he was not going to contest the leadership, he said: "I am going to be home secretary."
Last week, Mr Reid was at the centre of a row over sentencing after reminding judges about "existing guidelines" on sentencing, as it emerged that the UK's prisons were, in Tony Blair's words, "full to bursting point".
It prompted two Crown Court judges to decide against jailing sex offenders, blaming prison overcrowding.
LATEST HOME OFFICE PRESSURES
27 January...The News of the World claims 322 convicted sex offenders are missing across the UK
26 January....Home Secretary John Reid denies telling judges to give softer sentences to ease prison overcrowding
26 January....England and Wales Youth Justice Board head Rod Morgan quits over youth prisons' overcrowding
25 January....Risk of being a victim of crime in England and Wales rises for the first time since 1995, figures suggest
21 January....Proposals reveal the Home Office may be split in two to cover justice and security
14 January Senior civil servant suspended over failure to update police records of Britons convicted abroad
Mr Reid said that, under Labour, more serious criminals were being jailed for longer, or given indeterminate sentences.
He said 20,000 new prison places had been created since Labour came to power and said he had commissioned another 8,000 since taking up his job last May.
But sentencing guidelines had assumed a 15% reduction in trivial offenders being sent to prison as people did not want "to have to pay £40,000 a year to keep them in bed and breakfast", he said.
This had not happened, so he had made a statement to the National Criminal Justice Board, to remind them of guidelines - something which had been backed by the Lord Chief Justice.
Writing in the Guardian Mr Reid likened his Home Office reforms to renovating a house, saying that it is only when you take the wallpaper off "that you discover more problems".
He added: "These problems don't leave me beleaguered. If we weren't discovering more we wouldn't be reforming the Home Office. Indeed I expect more problems."
Meanwhile there is a dispute with prison officers over the plan to transfer staff to a new temporary jail due to open in Merseyside.
Brian Caton, general secretary of the Prison Officers Association (POA), told BBC Radio 4's Today programme that an extra 1,000 prison officers were needed.
Mr Caton said that Mr Reid and his predecessor "have done nothing to recruit staff to staff it, and now what they intend to do is pull staff away from existing prisons which are short of staff anyway....that is just unacceptable".
For the Conservatives, the shadow chancellor George Osborne, said Mr Brown's "Treasury targets, his skewed PFI accounts and his extraordinary decision to freeze the Home Office budget, have all contributed to the current crisis in our prisons and the criminal justice system".
For the Lib Dems, home affairs spokesman, Nick Clegg, said: "The British public don't want Bob the Builder as their Home Secretary. They want someone who is big enough to admit that the collapse in almost every aspect of our criminal justice system is a direct consequence of 10 years of mismanagement by New Labour."