Matt Hancock is facing a court challenge to stop him sharing sensitive NHS patient data with private firms.
Information on mental and sexual health, criminal records, abuse and drugs use is to be transferred to a new database that will be available to academic researchers and businesses.
Furious privacy campaigners have accused Mr Hancock is trying to “sneak out” a “secret” plan. They have served notice that the Health Secretary will face legal action unless it is put on hold.
More than 55 million patients have just until 23 June to opt out of the plan, which has had barely any publicity.
NHS Digital, which oversees the project, insists all details on the new database will be anonymous and protected by encryption security systems.
But lawyers for campaigners Foxglove and patients group JustTreatment say it is illegal with no guarantees that highly personal information won’t end up in the hands of companies that could use it commercially.
Cori Crider, director of Foxglove, said: “This is being rammed through in a rush with no c,Arita for patients or who gets the keys to this giant new data vault.
“If you ask patients whether they want details of their fertility treatment or abortion or results of other procedures shared with companies they are not going to be happy.”
A similar scheme was proposed seven years ago but dropped after a public outcry.
The new plan was announced with no fanfare by Mr Hancock last month and publicised only on the NHS Digital website and flyers in GP surgeries.
Mr Hancock was given a deadline of Friday night to respond to the threat of legal action but failed to do so.
Patients who want to opt out have to fill in a form and take it to their GP before their historical and future records become irreversibly part of the new information data set.
Phil Booth, founder of privacy campaigners MedConfidential, said: “They are trying to sneak it out, with only six weeks left before everybody’s records are transferred.”
He added that because the NHS has “opaque” commercial relationships it would be difficult to trace who ultimately views the data.
A spokeswoman for NHS Digital insisted no individual would be identifiable under the “very secure”new system and that it would be used only by carefully scrutinised organisations who can show a “legitimate and appropriate” legal basis for needing to use it.
She said GPs are already able to share data.
She added: “We have engaged with doctors, patients, data, privacy and ethics experts to design and build a better system for collecting this data.
“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.”