Three of Wales' seven health boards say they have about 720 patients well enough to leave hospital.
It said the NHS was facing exceptional pressure and there were more than 500 confirmed Covid cases in Welsh hospitals, with rapid increases in other respiratory viruses.
The letter added that it offered support and understanding, as this necessary adjustment in the clinical risk threshold for hospital care may be concerning for some professionals. Swansea Bay health board said it had nearly 280 patients who were medically well enough to leave - the equivalent of 10 wards - but could not for a number of reasons.
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