Immigration rules are hampering the ability of the NHS to recruit doctors, health leaders are warning.
The number of skilled non-EU workers granted UK visas is capped – with the Home Office arguing the restriction is in "the national interest".
But NHS bosses say increasing numbers of doctors are being refused permission, worsening rota gaps and the waits patients face for treatment.
In the North West a long-running scheme has been refused 100 doctor visas.
The scheme supplies junior doctors to more than 30 NHS trusts and bosses in the region have written to Health Secretary Jeremy Hunt and Home Secretary Amber Rudd asking them to urgently look into the problem.
Jon Rouse, chief officer of the Greater Manchester Health and Social Care Partnership, which represents services across the region, said he was shocked and concerned by the development.
"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."
He said these doctors would have worked across hospital services – and warned rotas would now go unfilled.
The NHS is already short of doctors, with a report from the regulator NHS Improvement warning one in 11 posts was vacant.
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Danny Mortimer, chief executive of NHS Employers, said he had heard of another 400 cases of blocked visas since December.
"We have examples of clinics being cancelled and delays in terms of patients receiving care.
"It exacerbates pressures in what are relatively small medical teams."
It is unclear exactly why this has happened. There is evidence in the NHS that it is getting harder to recruit and retain EU staff, putting pressure on non-EU sources.
The skilled worker cap applies to all sectors so the numbers the NHS can recruit is also influenced by other sectors from IT to banking.
Nurses have also been placed on the shortage occupation list, which means they are given a priority in international recruitment.
A Home Office spokeswoman said the system was in the "national interest".
She said any Tier 2 applications – as they are known – which are refused in oversubscribed months could reapply in future ones.
And she added there were a range of specialities, including A&E and radiology, that were on the shortage occupation list too.