The reforms to prison and probation services that are set to come in with the creation of the National Offender Management Service have attracted plenty of criticism.

Variously referred to as "a fabulous juxtaposition of the absurd and the insane" (Napo), and a "ghastly Orwellian concept" (Bob Andrews MP), Noms could be facing a shake-up following the resignation of chief executive Martin Narey. Certainly, many of those with an interest in the sector hope so.

Chris Stanley, head of youth crime at charity Nacro, says: "It would not be an exaggeration to call Noms a confusing mess. The government could use Narey's October departure to rethink Noms, but it wouldn't scrap it because that would be too embarrassing. Whoever replaces Narey must have a vision."

While there has been a broad welcome for Noms's aim of reducing reoffending by providing "seamless" supervision of offenders, the proposed top-down restructuring of the system has raised many concerns.

Noms was established with the aim of creating a single body with accountability for reducing reoffending and commissioning national services.

Under Noms, the probation service's commissioning powers are to be transferred to 10 regional offender management boards, which are due to take control of spending by April next year.

Since its inception, Noms has come under fire over its plans to encourage "contestability", making the probation service compete with private and voluntary sector organisations to run services, a move campaigners fear could lead to increased privatisation.

There is also anger over a perceived lack of consultation and parliamentary scrutiny.

Neil Gerrard MP, chair of the parliamentary justice unions group, says: "Narey's resignation has left a huge hole, but the best thing the Home Office could do is bring everything to a halt and take stock."

The Home Office is considering concentrating its efforts to reduce reoffending among 18 to 20-year-old prisoners under Noms.

In advance of the, as yet unscheduled, national roll-out, it has also published its first pathfinder study to explore how the model might work.

The pathfinder in north west England looked at how probation officers would adapt to their new roles under Noms as "offender managers", providing "end-to-end" supervision in both prison and the community.

While Narey, in his preface to the study, says it is "too soon" to assess the effectiveness of the model, the findings echo many critics' concerns.

The study, which focused on 18- to 20-year-old prisoners at Lancaster Farms and Thorn Cross young offender institutions, finds that "significant pressure" on YOIs to free up spaces for remand beds is hindering continuity of supervision for young offenders.

Cheryl Gillan, Conservative shadow minister for home and constitutional affairs, warns that the evidence shows the government can't "magic up" an end-to-end system for people who are being "constantly moved around the prison estate because of overcrowding".

Julian Corner, chief executive of charity Revolving Doors, says the pathfinder shows that reducing the prison population is "utterly critical" to the delivery of Noms.

Prison numbers have reached a record 76,506, according to the most recent Home Office figures, published last month.

But Home Office projections show this could top 91,000 within five years, compared with the government target of hitting a ceiling of 80,000 by the end of the decade.

Corner also points to the potential difficulties of implementing the Noms model caused by the frequent moves of prisoners to prisons away from their home areas. "The pathfinder focused on the North West, where there is less importing and exporting of prisoners out of the region," he says. "But in places like London, where more than half of prisoners are sent outside of the area, it will be a challenge for offender managers to sustain relationships."

Offender managers will be required to visit prisons to attend case meetings, but the pathfinder raises concerns that staff - drawn from the probation service - lack the skills to work in a prison setting.

The study also points to the "potentially significant cost implications" of travel to prisons by offender managers if prisoners are placed far away from their home areas.

In Cumbria, estimated staff time and travel costs for covering sentence and planning review meetings in prisons amounted to £208,000 - almost 4 per cent of Cumbria's annual resource budget.

The study warns that the national cost implications could prove a barrier to the implementation of the model. Harry Fletcher, assistant general secretary of probation union Napo, says the findings show the government has "underestimated" how resource-intensive the Noms model will be. "They would struggle to roll it out nationally," he predicts. "Unless it has the resources it will fail."

Neil Gerrard MP claims that the Home Office "keeps refusing" to answer questions about the cost implications of Noms.

He says: "There is no idea of costs. After more than a year since Noms was introduced, it's starting to get a bit suspicious."

A Home Office spokeswoman says the government is undertaking further evaluations and keeping estimated costs of rolling out the pathfinder under review.

The pathfinder also finds probation staff are concerned whether they will be able to carry out the numbers of required visits to prison because of high caseloads.

Julian Corner predicts that the size of caseloads could affect whether offender managers will be able to meet the Noms ideal of a quality relationship between manager and offender. "As the pathfinder evidence shows, the relationship between the offender manager and the offender is going to be key to the success of reducing reoffending," he says.

"The organisational reforms will need to be at the service of empowering relationships - 90 per cent of the success will depend on the human factor."

The pathfinder also identifies gaps in provision to address offending behaviour, particularly anger management, alcohol and victim awareness programmes.

Fletcher says greater provision is needed for 18 to 20-year-olds. He adds: "If Noms is really going to make an impact, there needs to be massive investment in literacy, numeracy and social skills programmes."

Corner also warns that the new model could focus too readily on the easy-to-reach, leaving those with complex needs in the system at risk of reoffending because of lack of specialist provision.

"You can't have a one-size-fits-all system for offenders, especially the most chaotic. Many have drugs or mental health problems and are intensely resistant to mainstream approaches. They are not going to slot in easily."

There are concerns about staffing too. The pathfinder predicts that high sickness and vacancy rates mean services would "struggle to achieve the degree of continuity provided by the model", and Fletcher believes the model will not work without "hundreds and hundreds" more staff. He adds: "There is simply not enough staff, and the south-east in particular is struggling with recruitment."

But the key challenge will be winning over existing staff, according to the pathfinder study. It warns that "resistance of staff to making the necessary change and risk of staff attrition" could be barriers to taking the model forward.

Corner says: "There is a lot of anxiety among staff as there are no hard and fast answers at the moment, and, understandably, they want to see what their jobs are going to look like. It could be that Noms is the answer to all their prayers, but they will have to wait and see."

Cheryl Gillan blames staff resistance on "absolutely abysmal" communication from the government, while Neil Gerrard says: "How can Noms work when so much of its potential workforce is opposed to it?"

It is a question that Martin Narey's successor must not ignore.