Background: Recent UK ‘hostile environment’ immigration policies, including obligatory charging and sharing of confidential data between NHS Digital and the Home Office, have created an atmosphere of fear and exposed already highly marginalised and vulnerable groups to significant health risks by increasing barriers to accessing NHS care. Methods: This is a cross-sectional observational study of patients accessing healthcare at Doctors of the World (DOTW) in the UK. DOTW is a humanitarian organisation, providing care to those excluded from NHS healthcare. We aimed to describe population characteristics of individuals using DOTW services and identify groups at greatest risk of facing ‘hostile environment’-related barriers to NHS care, specifically being denied healthcare or fear of arrest. Results: A total of 1474 adults were seen in 2016. Nearly all were non-EU/EEA nationals (97.8%; 1441/1474), living in poverty (68.6%; 1011/1474). DOTW saw a large number of undocumented migrants (57.1%; 841/1474) and asylum seekers (18.2%; 268/1474). 10.2% (151/1474) of adults seen had been denied NHS healthcare and 7.7% (114/1474) were afraid to access NHS services. Asylum seeker status was associated with the highest risk (adjusted odds ratio (OR): 2.48; 95% confidence interval (CI): 1.48-4.14) of being denied NHS healthcare and being undocumented was associated with the highest risk of fearing arrest (adjusted OR: 3.03; 95% CI: 1.70-5.40). Conclusions: Our findings make visible the multiple and intersecting vulnerabilities of individuals forced to seek care outside of the NHS, underlining the public health imperative for the government to urgently withdraw its ‘hostile environment’ policies and address their negative health impacts.