Two helicopters providing the East of England Air Ambulance cover have been grounded but managers hope to be flying again later on Monday.
Anglia 1 and 2, operated from Norwich and Cambridge, were grounded on Friday afternoon when Sterling Aviation's air operator's certificate was withdrawn.
The service was suspended after what are described as "operational issues ".
Director Richard Robinson said a contingency plan was put into action but fog prevented aircraft flying.
"The two helicopters used as back up could not fly because of fog so there was no service over the weekend," he said.
He was not able to comment any further and Sterling is expected to issue a statement later.
Mr Robinson said: "The relationship with Sterling Aviation has been very successful.
"This is not an air worthiness issue. We are dealing with the issue internally and looking at all the options."
The East Anglian air ambulance service covers Bedfordshire, Cambridgeshire, Norfolk and Suffolk.