Complaints about treatment provided by GPs outside normal working hours have risen significantly.
The Medical Defence Union has been notified about 116 complaints in the first eight months of this year.
In 1997, just 21 complaints about out-of-hours care were registered by the MDU in the whole year.
Experts believe the increase may be due to patients being less likely to see a doctor they know out-of-hours.
So far this year, complaints about treatment received outside normal working hours account for 10% of the total number notified to the MDU by its members.
In 1997, they made up just 1.8% of the total.
Dr Karen Dalby, clinical risk manager at the MDU, said the rising number of complaints about out-of-hours care was surprising, given the relatively small number of these consultations that take place, compared to those during surgery hours.
She said: "We do not know for sure why patients are complaining more about out of hours treatment, but it could be because of the change in the way these services are provided.
"Over the last few years, more GPs have been switching to out of hours co-operatives - a group of GPs coming together to provide out of hours care in shifts - or deputising services.
"This means that patients will often be seen by, or talk to, a doctor who is not their usual GP and so may not know their medical history.
"Most out of hours doctors do not have access to the patient's medical records and must base any decision about the urgency of the case on the information they receive from the person who took the call."
At present, GPs retain overall responsibility for their patients, whatever out-of-hours arrangements they adopt.
But under the new GP contract, doctors will be able to opt out of providing out-of-hours care - with this responsibility passing to Primary Care Trusts.
This makes it likely that more care will be provided by co-operatives and deputising services in future.
The MDU said that some of these complaints could have arisen because of breakdowns in communication and could be avoided with the establishment of good communication links between GPs and out of hours providers.
Dr Dalby said: "For example, GPs may wish to tell their out-of-hours provider if a terminally ill patient is likely to need pain relief and out of hours services should report back to GPs about any treatment given or follow up that is required."
The figures are taken from a larger survey of GP complaints which shows that 1,701 members asked the MDU for help with complaints last year, three times as many as when the procedure was first introduced in 1996 when just 557 complaints were notified.
The survey also showed that during 2001 and 2002 around 90 per cent of complaints were resolved by the practice at the first stage, known as local resolution.
Only a small percentage of complaints progressed to the Ombudsman, the General Medical Council or became medical negligence claims.
Dr Dalby said: "It's very encouraging that almost 90 per cent of complaints we see are being resolved by GPs within their own practice.
"Our advice to members is that an apology, if appropriate, an explanation of what went wrong and an assurance that systems have been put in place to prevent a recurrence, will go a long way to resolving many complaints."