If an employee claims that their late return from holiday is due to illness, must the employer treat a foreign medical certificate as evidence of incapacity?

Generally, an employer should treat a medical certificate from a doctor, even if from another country, as evidence of illness, unless there is evidence that the certificate is not genuine or that the employee is not incapacitated.

For the purposes of determining eligibility for statutory sick pay, s.14(1) of the Social Security Administration Act 1992 allows employers to set their own rules on what evidence they reasonably require of employees' illness. There is no restriction within the legislation on accepting foreign medical certificates.