Immigration rules are worsening NHS staffing pressures by preventing doctors from overseas coming to Britain to work, health chiefs have warned.
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 have 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.
Chief executive of NHS Employers Danny Mortimer (pictured) told the BBC 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 that supplies 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 that there were 100,000 vacancies across England's 234 acute, ambulance and mental health trusts.
Doctors are classed as Tier 2 visa applicants by the Home Office.
A Home Office spokeswoman told the BBC any applications refused in over-subscribed months can be applied for again in future ones.
Copyright (c) Press Association Ltd. 2018, All Rights Reserved. Picture (c) NHS Employers.