Hundreds of patients have been referred to the wrong consultant at the wrong time, because of a glitch in the Choose and Book system.
NHS Connecting for Health believes that only a handful of patients have actually turned up to find that doctors were not expecting them. But as many as 340 are thought to have been given details of another patient's appointment by mistake.
It is likely that new referrals are also being affected because the cause of the problem is unknown.
Dr Richard Vautrey, deputy GPC chairman, warned that problems like this were denting GPs' enthusiasm for the booking system.
'As soon as Choose and Book shows it is a reliable, robust and easy-to-use system, more GPs will use it,' he said. 'Until then, practices will continue to make their own decision about it.'
He called on Connecting for Health to apologise to all affected patients. Connecting for Health delayed the release of version 4.0 of the Choose and Book system while it handles the problem.
A spokesman admitted that turning up at an appointment where they are not expected would 'be confusing for patients', but stressed this was only happening in a tiny minority of cases. Choose and Book is used for 70,000 referrals a week.
The spokesman said the agency was working with its IT suppliers to resolve the issue, and contacting the affected providers to ensure patients received the correct referral.
He added it would not affect the roll out of free choice of care anywhere in the UK.
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