A hospital has asked people not to come to its accident and emergency department until further notice, saying it is running out of oxygen.
Watford General Hospital has declared a critical incident and said any emergency patients should go to other hospitals with A&E units.
A spokesman said anyone with non-urgent issues could go to the 111 NHS website.
The West Hertfordshire Hospitals NHS Trust said more information would be released soon.
As of 17:00 BST on Thursday, 29 people had died at the NHS trust’s hospitals after being diagnosed with Covid-19.
The trust is responsible for Watford General as well as Hemel Hempstead and St Albans City hospitals.
Current NHS advice tells people with coronavirus or suspected symptoms to avoid hospitals and other medical settings like pharmacies.