I left the elderly man I was examining to answer my bleep. "Mrs Eldridge is saturating at 77 per cent on 10 litres of oxygen, doctor. She's going blue," the nurse told me.

I ran up three floors to find the patient gasping for breath, neck veins bulging, heart racing. The breath sounds on the right side of her chest were very faint. She'd had a tension pneumothorax, a life-threatening emergency. Her right lung had burst, and with every breath air was escaping into her rib cage, compressing the heart, veins and other lung.

I knew what to do - it's just that I had never done it before. I got a large calibre cannula (a needle sheathed in plastic tubing) and an alcohol wipe to clean the skin. There was no time for local anaesthetic; two nurses held her arms while I pierced the skin and muscle between the second and third ribs. There was a hiss of escaping air. Over the next two minutes, her colour returned, and her pulse and breathing came back to normal. Emergency over, I called my registrar to tell him I was setting up to put in a chest drain. Crisis resolved.

Under the Government's new system of training doctors, I would not have been allowed to perform this procedure. The training (details have yet to be finalised) will be modular, based on documented acquisition of "competencies", much like nurses' training. Newly qualified doctors will have to have their competency in procedures such as intravenous cannulations, venesection (taking blood) and bladder catheterisation signed off by a supervisor. They will then presumably be "covered" legally to perform these tasks.

Dubbed "run-through training", the system aims to train doctors faster (eight years' training after medical school, rather than 10 or 15) by making them specialise early. At the end of their training these doctors will not be consultants, but accredited medical specialists.

Until now, doctors at all levels have been expected to do what's necessary in dealing with emergencies on the wards, and call a more senior doctor when they need help. They are expected to take responsibility for the patient, and there is no get-out clause: if you had not been lucky enough to be shown how to do something, you worked it out. That is the point of medical school; if all else failed, a knowledge of anatomy and the instructions with the kit sufficed. This is clearly not ideal, but when patients are critically unwell, they rightly expect a doctor to do his or her best to save their lives.

With the launch of the Government's reform of medical training, called Modernising Medical Careers, my generation of junior doctors are contemplating forced early retirement in February. There is an information black hole at the centre of the agency in charge of the new system: applications are to be made in January and we still have no idea how many posts will be available.

What is clear is that many of us will be surplus to requirements. Estimates are that more than half of junior doctors seeking to go into higher training - about 11,500 - will probably not find training jobs next year. Senior house officers (junior doctors) will be overtaken by the new system's trainees, for whom the application process is tailored. This will leave us three choices: taking dead-end jobs with no career progression; taking our skills abroad; or leaving medicine altogether.

We are in our mid- to late twenties, and are the workhorses of the NHS. We see you if you end up in A&E, or in outpatient clinics, or have to be clerked in for an operation. We take blood, arrange tests, do ward rounds and treat those who become critically unwell. We work a lot of nights and weekends. We are in training to become GPs or consultants. Having worked solidly towards this goal for 10 years - first at medical school, then juggling postgrad exams with a heavy work-schedule - the rug is being pulled from beneath us. It's apt that I'm writing this against a background of sleep deprivation; the feeling of despair this generates perhaps approximates to morale in the junior medical ranks these days.

We have worked so hard. We have consistently come in early and gone home late to keep our patients safe in a disintegrating system, even as the hospitals fiddled the official hours of work in order to pay us less. It has been a catalogue of friendships lost, families forgotten, relationships neglected.

I'm not asking for sympathy, but am instead trying to explain the coming exodus. The interim BMA report on this issue last summer revealed that 58 per cent of juniors would consider going abroad if unable to find a training post, while 38 per cent countenanced leaving the profession.

Aside from the gripes of the doctors themselves, does any of this really matter? Surely a surplus of doctors is better than a shortage, and more specialists trained faster sounds good. There is the cost, of course: 11,500 doctors, trained at a cost to the taxpayer of a quarter of a million pounds each, but no longer working in the NHS.

There has long been a need for greater emphasis on doctors' training, but the new system does not address the problem. We don't receive enough training because there is no time. The new competency-based approach clearly reflects the increasingly litigious environment in which doctors work, and the health trusts' need to protect themselves.

But the Government's requisition of control of medical training from the Medical and Surgical Royal Colleges, and the resulting insecurity, is also the latest, most successful attack on the power of the medical profession. And it cannot go unnoticed that it comes at a time when we are in the surreal position that the British medical profession - perhaps the country's most conservative institution after the monarchy - is all that stands between a Labour government and the piecemeal sell-off of the National Health Service.

From where I'm standing, it looks bleak. Thousands of us are contemplating career oblivion, and the lucky few who make it through face a future in medicine that would have been unthinkable only a few years ago: their practice limited to a few specialised things they're accredited for, and allowed to do for patients; even more managerial interference in patient care; dwindling resources; and an ever-increasing corporate presence in what will become a two-tier health service.

If I were Mrs Eldridge, I wouldn't fancy my chances in a system where the first doctor available may or may not have had their form signed for needle thoracocentesis.

Dr Lucy Chapman (not her real name) is a junior doctor working in an inner-city NHS hospital