The London Evening Standard reported that the animal is believed to have climbed into the roof at the North Middlesex University Hospital in Edmonton while the work was being carried out.
The inpatient ward was closed for several days while the building was secured and repairs carried out.
The fox was properly removed by the RSCPA.
A spokesman said no one was harmed during the incident, which occurred in a “non-patient” area of the inpatient paediatric ward.
The trust said work had since been undertaken to improve the security of the 1970s tower block on its North London site, so that animals could not gain access.
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