The British Medical Association (BMA) claims the government is not taking data security breaches seriously after the details of 168,000 patients went missing.
City and Hackney Primary Care Trust has lost clinical information relating to 160,000 patients after a computer disc failed to arrive at St Leonard's Hospital in east London.
A Department of Health spokesman said it believes that a further 8,000 patients in total may have been affected, and only a small proportion of these involve any clinical data.
But Richard Vautrey, deputy chairman of the BMA's GPs' committee, said: "Patients need to be absolutely confident that the information that is held securely cannot be lost in some haphazard way."
He said the development is worrying, especially as the government plans to bring in a centralised NHS computer network, Connecting for Health, featuring every patient's records.
Dr Vautrey added: "It's vitally important that any development of centralised systems is done in a careful and measured way."
The other trusts involved are Maidstone and Tunbridge Wells, Bolton Royal Hospital, Sutton and Merton PCT, Sefton Merseyside PCT, Mid-Essex Care Trust, East and North Hertfordshire, Norfolk and Norwich and Gloucester Partnership Foundation Trust.