A Wigan doctor has raised concerns about restrictions on the number of workers coming from overseas to join the NHS.
Health chiefs have warned that immigration rules are worsening NHS staffing pressures by preventing doctors from abroad travelling to Britain to work.
Limits on the number of visas issued to doctors from countries outside the European economic area are contributing to rota gaps and delays in patients receiving care, they said.
Figures from a regulator released earlier this year showed high vacancy numbers for NHS staff.
The Home Office says its limits are in the national interest.
Dr Sanjay Arya (pictured), medical director at Wrightington, Wigan And Leigh NHS Foundation Trust, was among those speaking out about the problem.
He told BBC Radio 4’s Today programme that the visa refusals would have a “huge impact on patient care and on patients’ safety”.
He added: “I see it every day that there are gaps on the rota. The existing doctors are working long hours, which is very tiring and makes them an unsafe doctor.
“If we had the pool of highly qualified doctors coming from overseas, it is only going to help our patients and our NHS.”
Chief executive of NHS Employers Danny Mortimer said he had heard of 400 cases of blocked visas since December.
“We have examples of clinics being cancelled and delays in terms of patients receiving care,” he said.
“It exacerbates pressures in what are relatively small medical teams.”
Some 100 visas are reported to have been refused for a scheme in the North West supplying junior doctors to 30 NHS trusts.
Bosses are said to have written to Health Secretary Jeremy Hunt and Home Secretary Amber Rudd with their concerns.
Chief officer of the Greater Manchester Health And Social Care Partnership, Jon Rouse, said: “As we reach the end of a winter where the NHS has been stretched to its very limits, partly as a result of a lack of medical workforce, we find it almost impossible to understand how this decision can have been reached.”
In February NHS Improvement said there were 100,000 vacancies across England’s 234 acute, ambulance and mental health trusts.
Doctors are classed as tier two visa applicants by the Home Office.
A Downing Street spokesman said: “We estimate that around one third of all tier two places go to the NHS. We need to have a system which ensures the NHS gets the staff it needs. That’s why we are investing in the workforce and we have record numbers working in the health service.”
A Home Office spokesman said applications refused in over-subscribed months can be applied for again.