Leading doctors are calling for an immediate delay to a programme to move information from GP records in England to a central NHS Digital database.
Informing patients could not be left to busy GPs, the Royal College of General Practitioners (RCGP) have told Health Secretary Matt Hancock in a letter.
Campaigners say they have been flooded with requests to help patients opt out before the 23 June deadline.
But NHS Digital said: "Patient data saves lives."
The General Practice Data for Planning and Research (GPDPR) programme will transfer data from records created up to 10 years ago in "near real time".
And although NHS Digital itself proposed a delay, the Department of Health decided against one.
Officials stress GP data is already used for research, with approval, and has helped shape the response to Covid-19.
But Dr Farah Jameel, of the British Medical Association (BMA), said the timeline needed a "hard reset".
"NHS Digital and the government must postpone the date of the first 'extraction' of data - scheduled for 1 July - until such time as the public are able to make a fully informed decision about what happens to their data," she said.
NHS Digital, meanwhile, said it continued to engage with both the BMA and RCGP and was "exploring further options to expand our communications approach".
"We expect GPs to be ready to implement this new system from 1 July," it said.
And it would collect a range of information, including:
- data about "physical, mental and sexual health"
- details of gender, ethnicity and sexual orientation
The NHS says:
- the data will be used for planning and research purposes only
- each application to use it will need approval from advisory groups
It may not be used "solely" for commercial purposes.
But some private-sector organisations will be able to see it, with permission.
And critics point to the controversial involvement of US data company Palantir in the analysis of other NHS data.
The firm - sometimes described as the "scariest" of America's tech giants - is known for supplying data-sifting software to government agencies, and has been linked to to efforts to track undocumented migrant workers in America.
A petition organised by Open Democracy - part of a group of organisations mounting a legal challenge to the GPDPR - has been signed more than 16,000 times.
It calls on the health secretary to:
- drop the 23 June optout deadline
- hold a "proper" consultation
- not "to share our data with private companies for profit".
And on Twitter, the hashtag #Nhsdatagrab started to trend in the UK on Sunday, following an awareness push by campaigners.
Patients wishing to opt out must return a completed form to their surgery
And campaign group MedConfidential, which is printing out and posting forms to those who struggle to obtain them online or from the NHS, told BBC News it had seen a "massive increase" in requests, reaching a peak of 100 per hour, and filled its two nearest post boxes to capacity.