Thousands of prescription forms, carrying the names and addresses of patients, go missing every year as they are transported around the NHS.
The Conservatives claim the loss of prescriptions is "endemic"
The government has admitted that almost 300,000 have been lost or stolen in England since 1997.
Shadow health secretary Andrew Lansley said handling of personal information was "serially incompetent".
But Health Minister Dawn Primarolo said the number of prescriptions lost was less than 0.01% of the total issued.
The paper-based prescription system means that when issued to a patient, and handed over at a pharmacy, it must be sent as part of a batch to the Prescription Pricing Division for processing at one of their centres scattered around the country.
In response to a Parliamentary Question from the Conservatives, Ms Primarolo said that between April 1997 and September last year, a total of 298,100 forms went missing in 220 separate incidents of loss or theft.
There appears to be no year by year trend, either rising or falling - the highest number of forms lost was almost 69,000 in 2000/01, and the lowest 10,400 in 2005/06.
The government said that in each case, the postal service, courier or police had been notified.
However, Mr Lansley said the problem was "endemic".
"The government has been serially incompetent in looking after people's personal data," he said.
"This isn't just the odd mistake here and there, this is equivalent to 78 prescription forms lost every day since 1997.
"People need to feel like they can trust the government when they hand over personal data."
'No perfect solution'
A spokesman for the Royal Pharmaceutical Society - which represents pharmacists - said that prescription form loss, while rare, was inconvenient to pharmacists and could possibly see patient data falling into the wrong hands.
"There are electronic systems on the way, but no system would be perfect, and there would still have to be a paper back-up somewhere."
A spokesman for the Department of Health said that clear guidance on "information governance" was available to the NHS, and a further review of security was underway.
"The NHS takes the protection of patient data extremely seriously."
A spokesman for Connecting for Health, which is introducing new NHS computer systems, said that the number of forms sent from pharmacies was expected to fall dramatically with the introduction of an electronic prescription system.
"The GP signs a prescription electronically, repeat prescriptions will be dispensed electronically and pharmacists will be reimbursed for payment electronically," he said.
He said that the new system would be more secure than the current paper-based version.
Meanwhile, the government has announced that the price of prescriptions will increase by 25p to £7.10 from April in England.
It comes as Scotland has started phasing out charging for prescriptions, while Wales already provides them free.