Most patients will be able to see GPs only in office hours when new contracts for family doctors take effect in April.

Most patients will be able to see GPs only in office hours when new contracts for family doctors take effect in April.

Senior doctors warned that out-of-hours NHS services could be thrown into chaos by the reforms, which will allow GPs to opt out of working at nights and weekends. They said the change could lead to patients inundating casualty departments because their GPs were unavailable.

Companies which provide night and weekend cover for GPs said they might not be able to cope with the demand, after a survey showed that more than half of doctors plan to ditch out-of-hours working as soon as they can.

Family doctors now have a legal responsibility to provide out-of-hours care for patients. Some do evening and weekend shifts themselves, while others pay GP co-operatives to provide cover.

Under the new contracts, GPs will be able to hand responsibility for out-of-hours duties to the local primary care trust. Doctors who do so will forgo £6,000 a year from their salaries, and the money will go to trusts to spend on locums and out-of-hours staff.

Allowing GPs to opt out was a key concession by the Government when negotiating the new contracts. Ministers believed that many doctors would opt for the extra money and continue providing cover, rather than giving up the bonus.

However, a survey of more than 1,000 GPs by the medical newspaper Pulse found that 47 per cent intended to ditch out-of-hours work. Another 39 per cent plan to work just enough shifts with GP co-operatives to cover the £6,000 cost to them of opting out.

Dr Mark Reynolds, the chairman of the National Association of GP Co-operatives, said some out-of-hours companies would struggle to provide enough staff to cover all the opted-out practices. "All out-of-hours organisations are anxious about getting enough people to work," he said.

Doctors were also warned that their workload on Mondays would soar from patients unable to see their normal GP over the weekend.

Dr Laurence Buckman, a negotiator on the new contracts for the British Medical Association, said: "Over the weekend, things will brew."

Nine out of 10 GPs admitted that the opt-out policy would lead to increased pressure on accident and emergency departments.

More than a third believed the opt-out would lead to patients using the NHS less responsibly, either by resorting to the casualty department or by making inappropriate calls to the telephone helpline NHS Direct or the on-call service.

GPs were determined to win the right to the opt-out in the new contracts, claiming the on-call system meant they worked long hours and were often plagued by pointless calls and petty demands.

Doctors gave many examples of time-wasting calls out of hours. One said he had been asked for advice about a patient's dog; another had been asked to visit a woman because she had a spot on her face; and a third doctor had been asked to visit a patient's house and scare away foxes.

Under the new contracts, which are designed to stem the recruitment crisis in general practice and improve the lives of family doctors, some GPs could earn up to £100,000 a year by choosing to work only evenings and weekends as cover for other doctors.

Another proposal is to recruit local pharmacists to help to provide some out-of-hours cover.