Healthcare workers' union Unison has warned that the loss of European employees would leave the NHS in "a state of near collapse".
Between December 2014 and November 2015, the equivalent of 93 EU citizens started full-time jobs at the Lancashire Teaching Hospitals NHS Foundation Trust, NHS Digital data shows.
But over the same period in 2017-18, after the UK voted to leave the EU, the trust hired just 33 EU citizens.
Sara Gorton, Unison's head of health, said: "Departing EU nationals mean serious problems for the NHS.
"Brexit is making it harder for hospitals to recruit, and causing workers to question staying here.
"Without the many health employees from across Europe, the NHS would be in a state of near collapse, and their skills and expertise have helped limit the effects of the huge staffing gaps.
"Further staff losses would mean even more stress for an already overstretched workforce, and would have a devastating impact on patient care."
While the number of EU nationals joining the staff of the Lancashire Teaching Hospitals NHS Foundation Trust is falling, the opposite is true for workers from the UK.
Between December 2017 and November 2018, 724 full-time UK employees joined the trust, 71 more than in 2014-15.
Across England, fewer EU nationals are joining the NHS and more are leaving than before the referendum, while the trend is reversed for staff from the UK.
The number of EU citizens starting full-time jobs has fallen by 26 per cent, from 14,500 in 2014-15 to 10,800 in 2017-18, while the number leaving work has risen from 6,700 to 9,600.
Conversely, 109,000 full-time UK workers joined the NHS in 2017-18, compared with 103,000 three years earlier.
The Department of Health and Social Care said it was encouraging NHS workers from the EU to apply for settled status.
A spokesperson said: "EU workers play a vital role across the health and social care system, and we want them to stay here long after the UK leaves the EU.
"Our priority is to make sure that high standards are maintained across the healthcare system, and that patients continue to receive the high-quality care they deserve."