Up to three million pensioners will be unable to stop their personal health details being sold to firms, warn campaigners.
Health Secretary Matt Hancock wants to pool data on 55 million NHS patients, including sexual histories, mental health, criminal activity and drug abuse.
NHS campaigners say it would put sensitive records at risk of privacy breaches and commercial exploitation.
Mr Hancock aims to offer only an online opt-out and is resisting calls to notify every NHS patient by mail.
There are fears up to three million over-65s with no computer access will be unable to protect records.
Tory MP and ex-Cabinet minister David Davis said: “It’s not safe to pool this data and it’s probably not legal.
“People should have a choice about participating, so the Department of Health should write to people. If they refuse we will fight until they do.”
NHS Digital, overseeing the project, insists all details on the new database will be anonymous and protected by encryption security systems.
Lawyers for campaigners Foxglove and patients’ group Just Treatment say it is illegal.
Foxglove’s Cori Crider, said: “This is being rammed through in a rush with no clarity for patients or on who gets the keys to this giant new data vault.”
A similar scheme proposed eight years ago was dropped after a public outcry. That plan included consulting every patient.
Protests have already forced Mr Hancock to postpone the deadline for opting out of the new scheme from last week to September.
A NHS Digital spokesman said: “The data will only be used for health and care planning and research purposes, by organisations which can show they have an appropriate legal basis and a legitimate need to use it.”