The NHS has been told to "immediately stop sharing patient data with the Home Office" for the purpose of tracing immigration offenders.
Dr Sarah Wollaston, who chairs the House of Commons health select committee, has criticised a deal between NHS Digital and the Home Office that may be putting pregnant women and modern slavery victims at risk.
She said it was "deeply concerning" that NHS Digital "ever chose" to breach the patient confidentiality standards in order to assist immigration investigations.
MPs were previously told that while patient confidentiality can be breached in cases of serious crimes such as rape and murder, it should not be breached for immigration offences.
Earlier this month, health experts told Parliament's health committee of the dangers of the data-sharing agreement, saying many migrants in need of healthcare are too scared to see a GP for fear of deportation.
Migrant domestic workers in abusive employment have suffered because they were too frightened to seek medical help, according to Marissa Begonia, the coordinator at Voices of Domestic Workers, with at least one recorded death.
Some are turning up at A&E wards, where they do not need to hand over personal information to receive treatment – putting additional pressure on the health service, the committee heard.
Data published in October revealed that police are failing modern-day slavery victims, with a conviction rate of less than 2% for the 1,265 offences reported between April 2015 and March 2017.
This month, banks also began to undertake immigration checks on millions of accounts as part of a Home Office scheme to create a "hostile environment" for people living in the UK unlawfully.
Dr Wollaston MP, who a Conservative party member and former GP, tweeted: "It is essential that all patients can trust that their confidential personal information, including their address, will not be shared without consent.
"Only in exceptional circumstances should that principle be breached and after full consideration of the public interest."
NHS Digital told Sky News: "We can confirm we have received a letter from the chair of the Health Select Committee. We will consider it carefully and will respond fully in due course."
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A spokesperson for the Home Office added: "Non-clinical information is shared on occasion between health agencies and the Home Office to locate individuals suspected of committing immigration offences – this data is strictly controlled and only shared if there is a legal basis to do so."
They added that the data-sharing agreement "simply streamlines existing processes and does not involve additional data being provided to the Home Office. Urgent and emergency care is always available from the NHS regardless of immigration status".
"We will carefully consider the points raised by the Health Select Committee."