The MP, who was Conservative shadow home secretary until only a month ago, said the party would be "mad" to hand over patient health records to Microsoft or Google to hold online, if the party won the next general election.
The criticisms were made in an opinion piece that Davis wrote in The Times, in which he responded to his party's plans to scrap the central government 'spine' of patient health records, which is part of the £12.7 billion National Programme for IT. Conservative leader David Cameron had said the measures were a possible alternative to the current setup.
While there were "massive weaknesses in the NHS' bloated central database", he said, there are also "enormous risks" from using the private sector to perform the same function.
The dangers lay in potential misuse of data by companies with a large internet presence, he claimed.
Google came under the heaviest criticism from Davis. He cited human rights watchdog Privacy International which gave the company a low privacy rating.
"Google is the last company I would trust with data belonging to me," he wrote. "My party would be mad to give control of sensitive records to an internet giant notorious for ignoring privacy concerns."
The government needed to "impose some limits on how personal data is managed, anonymised and used", he said, admitting that this would be a difficult task that required international co-operation. The data also needed to be held in the UK to ensure it is protected by British law, he said.
Davis expressed concerns about how the IT suppliers would make money if the service were free to users. "It should not be possible to make money out of holding health data," he said. "Health information has to be secure, and should not be available to be used for commercial purposes."
"That means it should not be sold on, it should not be data mined for commercial insights, and it should not be used for targeted advertising," he said.