NHS trusts are increasingly employing key clinical staff on "zero-hours" contracts which threaten to turn parts of the reformed service into an army of "temps".

Hospital trusts and private firms are turning to the contracts – which bind employees to on-call working but do not guarantee any specified number of hours or income or employment rights – to meet demands in the Government's controversial market-driven changes to the NHS, The Independent has learnt.

The move risks damaging training and expertise, critics say, and could risk a "G4S-style" situation where trusts could find that supposedly key employees are not available to work.

Zero-hours contracts have been legally used before, predominantly in the service sector and on the low-paid fringes of the health service. However their increasing use in core services such as cardiac, psychiatric therapy, respiratory diagnostics and adult hearing is a key change to the fabric of NHS employment.

One newly qualified student contacted by The Independent, who wants to become a radiologist, said she had been asked to consider working on a zero-hours basis for a number of trusts. Preferring not to be named "because this could damage my career", she said: "It [the contract] offered me no career security at all. No one could tell me how I would be able to continue my training."

The public service union, Unison, said it has received "worrying feedback" on zero-hours contracts as the Government's reform of the NHS sees hospital trusts and private firms bid for patient services. Unison's senior national officer, Sara Gorton, said that those trusts and private firms chosen to provide health services under the new system "do not know how much work will be given, because it is supposed to be up to patients to decide which provider they choose.

"Yet in order to be approved as a provider, they must demonstrate they can operate with spare capacity so they can meet increasing patient demand should that happen. But rather than simply hire staff that might be needed, which is expensive, many providers are turning to zero-hours contracts. The whole thing is an unnecessary, untested experiment – a nightmare."

Trusts that use the contracts include Oxford Radcliffe Hospitals, South Gloucester and University Hospitals Bristol.

The shadow Health Secretary, Andy Burnham, said: "I'm calling on the Government to halt the spread of zero-hours contracts in the NHS pending an urgent review into the potential risks to continuity of care and patient safety."

Mr Burnham said that the growing use of the contracts was a "depressing symptom of the Government's drive to turn England's health service into a full-blown market".

Oxford Radcliffe Hospitals describes "zero hours" as "as-and-when" contracts that do not have any contractual hours. The trust says this lets it have a "pool of experienced staff that are able to help out at times of increased demand".

South Gloucester Primary Care Trust uses the contracts as part of its NHS flexible staffing "bank system". Although the "as-and-when" contracts are used by some staff there to bring in additional hours that supplement their other work, other experienced healthcare professionals within the trust are on permanent zero-hours contracts.

University Hospitals Bristol NHS Trust now operates a temporary staff bureau with more than 1,200 people on its books employed under zero-hours contracts, in clinical, science, medical, dental and nursing posts.

Zero-hours staff, the Bristol trust said, underwent the same employment checks, induction and training updates as full-time employees, adding: "They are employed to fill vacant shifts when these arise and are paid an hourly rate for the hours they work."

The system, the trust added, saved public money by reducing the need to use more expensive agency staff.

A consortium of 20 health trusts in South-west England, which is threatening to abandon the NHS's nationally negotiated pay and conditions framework and install its own pay structure, has also discussed extending the use of zero-hours contracts to help cut its wage bill.

Circle, the first private healthcare provider to run an NHS hospital, Hinchingbrooke in Cambridgeshire, was contacted by The Independent and asked if it used zero-hours contracts to help it win new NHS contracts by proving it could meet unexpected demand. Despite being given numerous opportunities to respond a spokeswoman for Circle said yesterday that she "did not know" if it used zero-hour contracts or not.

Mr Burnham said "What is going on here is the unpicking of the fabric of the employment system of the NHS in England. Zero-hours contracts have previously operated safely within the NHS. They suited some staff willing to trade a bit of uncertainty for extra pay. But it's very different to extend these contracts into core delivery services."

Staff hired by qualified providers on a zero-hours basis simply to prove they have spare capacity was "a dangerous model to provide healthcare" said Mr Burnham, adding: "It is the casualisation of our health service, turning parts of the NHS into a 'temping' workforce." Comparing the recent fiasco at the Olympics which saw the private security firm G4S unable to deliver crucial staff on the eve of the Games, Mr Burnham claimed that providers using zero-hours contracts may "claim they have all these staff ready to go. But the reality might be different. Hospitals could face a 'G4S' situation, with the taxpayer picking up the bill again, or worse, patients being left untreated."

The Department of Health maintains that providers "do not require available extra capacity to meet unexpected demand". A spokesman said: "The NHS has always had to be flexible to meet variations in demand and it is up to providers to organise themselves to make sure this happens."

Rules for qualified providers only applied to the introduction of community and mental health services, a Department of Health spokesman added.

McJobs? the controversial terms

James Cusick

The standard business definition is an arrangement between an employer and employee who has agreed to be available for work "as and when" required – but without any particular hours being specified or guaranteed.

It offers no fixed period of working time, nor does it guarantee a level of salary.

Regardless of the uncertainty, the employee is expected to be available on-call and will only receive payment for the number of hours worked, not the number of hours they have waited to work. The minimum wage rate applies to the arrangement.

Sometimes referred to as being "in work, but not always at work", zero-hours contracts have traditionally been associated with service and retail sector jobs that reflect seasonal changes in demand.

McDonald's, which employs 87,500 staff across 1,200 restaurants in the UK, says that the majority of its staff are employed on an hourly basis, which it insists suits those looking for flexible working. The company would not reveal how many of its shift workers were employed on zero-hours contracts, though the number is thought to be high.