It is "inevitable that things will go wrong" when prisoners are freed early to try to alleviate prison overcrowding, the Chief Inspector of Probation has said.
About 5,500 prisoners in England and Wales are expected to be released earlier than planned in September and October as part of the temporary scheme.
It does not apply to those convicted of sex offences, domestic abuse, terrorism or some violent offences.
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Those released early will serve the rest of their sentence under the "strictest licensing conditions" and will be tagged, the government has said.
But Martin Jones, who became Chief Inspector of Probation in March, said there are "no risk-free options available".
He said the eight weeks the government has given the probation service to plan for the scheme has given it "at least a fighting chance of getting this right".
However, he warned the number of offenders being released means some could reoffend when they should have been in jail.
He told The Times: "I think it's inevitable, being realistic about it, that things will go wrong. I wish we could live in a perfect world where that doesn't happen.
"What I think you should start to see, at least, is that if people have to focus on those, that they start to identify where things go wrong, and they draw lessons from that quite quickly.
"I also think there's a little bit of a numbers game to some extent, you're rolling the dice all the time in relation to serious further offences.
"You know, ultimately, if you release thousands of people, a number of those cases will ultimately, sadly, there will be things that will go wrong."
A Ministry of Justice spokesman said: "The new government inherited a justice system in crisis and has been forced into taking difficult but necessary action to ensure we can keep locking up dangerous criminals and protect the public.
"Anyone released into Home Detention Curfew is risk-assessed, faces the strictest licensing conditions and must be tagged."
Sky News reported earlier in the week there are only 100 spaces left in male prisons across England and Wales.
It is the closest the system has come to running out of places.
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Shortly after becoming prime minister earlier this summer, Sir Keir Starmer approved plans for some prisoners to be let out after serving 40% of their sentence - rather than the usual 50% - which will come into force on 10 September.
In the wake of the nationwide riots after the Southport stabbings, the Ministry of Justice also activated Operation Early Dawn, allowing defendants waiting for a court appearance to be held in police cells for longer until space is available.
But last weekend's events, including Notting Hill Carnival, have pushed capacity even closer to the maximum.
Sir Keir said on Wednesday, during a visit to Berlin, more prisons will be built to manage overcrowding once the government gets its "hands on the planning laws".
He said the lack of new prisons is one of the reasons for the current capacity crisis.
The prime minister, a former director of public prosecutions, previously said the decision to release prisoners early "goes against the grain of everything I've ever done".
Labour has repeatedly accused the Conservatives of neglecting the justice system during their time in office, saying prisons were at risk of overflowing.
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