A data-sharing agreement between the Home Office and the NHS to track immigration offenders has led to people becoming "too frightened" to access healthcare, MPs have heard.
An agreement used by the Home Office to make requests for non-clinical data about suspected immigration offenders from health bodies was published in January last year.
The "memorandum of understanding" sets out how information is shared as part of efforts to trace those in the country unlawfully.
The Health Select Committee heard that some undocumented migrants are avoiding seeking healthcare as a result.
Marissa Begonia, coordinator of the group Voices of Domestic Workers, told MPs that people had died after avoiding getting help.
"We have one member who died, and never sought any hospitalisation or GP because she was too frightened," she said. "What killed her was she was frightened to access healthcare."
Dr Lucinda Hiam, a general practitioner working for Doctors of the World, added: "In our clinic in particular there are pregnant women who are too frightened to access healthcare and that is something that really worries us.
"In one extreme circumstance we had a woman present in labour because she was too scared to go to hospital.
"In our clinic, the average time people have been in the UK before even trying to access healthcare is six years so when they present, often with conditions like diabetes and high blood pressure which can easily be managed, they're out of control. We had to send 45 people to A&E last year."
She added that since the memorandum was made public, Doctors of the World had to change the advice to patients in its clinic.
"We still do tell patients to register with a GP but we have to say that 'your data might be shared' and a lot of people now are not going," she said.
Dr Hiam added: "I saw a woman a few weeks ago from Eritrea who had been living in this country for seven years and being kept as a slave and subjected to horrendous sexual violence. She didn't feel able to go to a GP so that's the atmosphere of fear we're creating."
Dr Joanne Bailey, member of the advisory panel for the National Data Guardian, Dame Fiona Caldicott, told the committee: "The policy has obvious consequences for undocumented migrants and also for the wider health.
"Her concern is that if this approach that is taken with these data releases were extended to other parts of the NHS, or to other crimes of a comparable gravity, patients may well come to feel that the NHS family is disclosing their information on too low a threshold or without strong enough safeguards.
"Dame Fiona is concerned about the broad public trust implications on care, on patient safety and on the wider health system."