The health ombudsman has voiced concerns about the quality of care given to NHS patients out-of-hours.
Many people need to see a doctor out-of-hours
Ann Abrahams said she had investigated a number of complaints against agencies that provide care to patients when GP surgeries close.
"I have seen a number of serious cases involving GP deputising services," she says in her latest report.
"Quality of care, communication with patients and record-keeping in these cases is a real cause for concern."
Her comments come as more GPs are handing over responsibility for out-of-hours care to primary care organisations.
Some of these are in turn contracting services out to other NHS bodies or private firms.
Under their new NHS contract, GPs no longer have to provide care to patients in the evenings and at weekends.
The ombudsman's report covers the six months before the new GP contract came into effect, between October 2003 and March 2004.
It includes details of four complaints involving out-of-hours care. Three of these centred on Primecare, a private firm employed by the NHS in some parts of the country to provide out-of-hours care.
In one case, the parents of a 32-year-old man in north-east London, known only as Mr Y, phoned Primecare one evening at 2200 after his "flu-like" symptoms got worse.
The parents phoned again shortly after midnight, saying their son was seriously ill.
When a doctor finally arrived at 0344, Mr Y was delirious, vomiting and had a non-blanching rash all over his body, which did not disappear when pressed.
The doctor phoned 999 and arranged for an ambulance to transfer the man to hospital. He wrote two letters of referral and left before the ambulance arrived.
The ambulance arrived at 0446 and Mr Y was taken to hospital, where he died shortly afterwards from meningococcal septicaemia.
The ombudsman criticised the GP, saying he should have given Mr Y an antibiotic.
She added: "The GP further failed in his duty of care by not waiting until the ambulance arrived."
Primecare has since pledged to introduce new rules on the handling of calls and cover during busy periods. The doctor is receiving extra training.
In another case, a man in Middlesbrough telephoned Primecare at the beginning of a weekend after his wife, known only as Mrs W, experienced pains in her arms and chest.
The woman had non-insulin diabetes and heart disease and was waiting to have a heart bypass operation.
She was assessed by a doctor at a Primecare centre, prescribed ibuprofen and sent home.
However, her pain got worse the next day and Mrs W returned to the centre. She was told by another doctor to take stronger painkillers.
She visited her own GP the following day who told her she had suffered a heart attack over the weekend. She was admitted to an intensive care unit that day.
The ombudsman said doctors might have considered that Mrs W had suffered a heart attack, given that she had heart disease and was awaiting surgery.
She also criticised Primecare's handling of Mrs W's complaint about her treatment.
In a statement, Primecare said it had introduced changes to its out-of-hours services.
"We have responded in detail to the specific cases during the course of the ombudsman's investigations and have already put in place changes to procedures and services in line with the findings of these cases."
It added: "The cases in the report occurred some time ago and it is important to note that much has changed in out of hours care over the last 12-18 months."