Casualty departments around Britain are in danger of being overwhelmed by the number of patients turning up in the evening when thousands of family doctors stop providing after-hours cover this spring.

From April, GPs will be allowed to opt out of seeing patients in the evening and at night with the agreement of their local primary care trust. There are concerns, however, that their managers are struggling to make alternative arrangements because of the huge shortage of medical staff and budget pressures.

Up to four-fifths of GPs plan to give up responsibility for providing out-of-hours cover in return for a £6,000 pay cut - a deal that was hammered out with Health Secretary John Reid as part of modernisation reforms. From December, they can opt out even if their managers have no arrangements in place.

But there is growing uncertainty about how the trusts will offer a safe level of medical cover. Many managers hope to rely on nurses, paramedics and community pharmacists to take calls in the evening and to do home visits. Not only are there too few doctors to do the night shifts, but those who hire themselves back for after-hours work may prove too expensive, as they plan to charge between £40 and £80 an hour for the extra work. Some trusts are intending to send patients into local casualty departments, which will have to employ extra staff to cope with the influx.

For many GPs, however, this is not an attractive option. Louise Dale, a GP in Retford, Notts, has said she will resign rather than cover a casualty department at night, where she will be swamped with work.

'The final straw came two weeks ago, when managers asked me to man a casualty department at night,' said Dale, a 42-year-old mother of two. 'In the past I've been part of a co-operative and my nights on duty have been busy but not unmanageable. Working in casualty and seeing everyone who wasn't an emergency would be enormously pressurised. Why would I want to do this after a long day at work?'

Dale, who has written children's books and now intends to devote herself to it full-time, said: 'The goodwill that has held the NHS together for years is disappearing. I don't think the Government realises how many doctors like myself intend to leave. It's something they would rather not think about.'

Hospital doctors are equally worried that the change will put them under impossible pressure at night. John Heyworth, head of the British Association of Accident and Emergency Medicine, said: 'We are very concerned that emergency departments will be swamped by additional patients. The Government needs to deal with this - and quickly.'

Mike Farrar, the chief contract negotiator for the NHS Confederation, said that, although there were anxieties about the new system, patients could be assured that they would continue to receive a high standard of care around the clock.

The British Medical Association argues that an overhaul of hours is overdue as part of modernisation. Hamish Meldrum, deputy chair of the BMA's GPs' committee, said: 'There is a critical shortage of GPs, but in my area half the doctors still plan to work out of hours. The 24-hour responsibility for patients and the increasing workload has made the profession unattractive; we have to address that.'

Meldrum said that between £50 and £60 an hour was the figure most doctors were talking about for the out-of-hours work. 'It may sound a lot, but you wouldn't get a plumber to fix your drains for much less.'

Underlying the issue is the growing shortage of family doctors, with more than 3,000 GP posts vacant. The problem of doctors like Dale retiring early has put more pressure on the existing doctors, who complain about the bureaucracy and the target-obsessed culture of the service.

The Government has made an extra £28 million available, primarily to ensure that rural and inner-city out-of-hours services are provided, in addition to the £110m a year which is being set aside to develop out-of-hours cover.

But patients' groups worry that there cannot be any reassurance about the quality of the care until they see how it works in practice. A spokesman for Help the Aged said: 'Anything that decreases the availability of medical care for elderly people is obviously dangerous. We would want absolute assurances that there would be no decline in the quality of the care. Half our problem is that this uncertainty will create anxiety for older people, and that is harmful in itself.'