Efforts to solve the prison overcrowding crisis will not work without a major injection of funding in the upcoming Spending Review, a union has said.
The review of sentencing policy announced last year, which is being led by former justice secretary David Gauke, "may come to little effect", according to Ian Lawrence, general secretary of Napo, which represents probation and family court staff.
"I'm struggling to see how a package of sentencing reform can work without the necessary support," he said.
Speaking to the BBC, he said probation staff are already "overworked" and that any "cost cutting" could increase pressure on services.
He said a funding cut for the Ministry of Justice in SR25 could lead to cuts in probation services – and therefore less support for offenders in the community.
"In other words, they go out of prison and they've got no option but to commit crime because they have no means of supporting themselves," he said.
"They're back in prison within weeks. And so it goes on and that costs the taxpayer millions."
Napo is currently in dispute with HM Prison and Probation Service over pay progression and workloads and has submitted a claim for a 12% pay rise for probation staff this year.
He said strike action could be on the cards if the next pay offer is too low.
"We think senior leaders in [the service] have a responsibility to let ministers know the gravity of the situation," he said.
"And that worries me as to whether ministers are truly sighted on the operational crisis that exists in probation right now."
Gauke's review is expected to be published this month, before the Spending Review on 11 June.
He is reportedly thinking about recommending that short prison terms be scrapped.
In an interim report published in February, Gauke said there had been an underinvestment in probation and other services that can provide rehabilitation and reduce reoffending because resources had been diverted towards the increasing cost of rising prisoner numbers
He said that without radical changes, prisons in England and Wales could run out of cells by early next year.
Lawrence said his union would "welcome any initiatives to reduce the numbers of people in our prison estate".
"But that can't come without the lord chancellor absolutely recognising the pressures that the probation service is now facing and will in the future," he added.
"And that's why we need brave, bold investment decisions by this government and not more of the same."
A Ministry of Justice spokesperson said a "crisis" in the justice system had "put a huge strain on the probation service".
"We are hiring 1,300 new probation officers, investing in technology to cut back on admin, and increasing focus on those offenders who pose the greatest risk to the public," the spokesperson said.
"This will ease pressure on the service, help cut reoffending and keep our streets safe."