According to the Ana Aeroportos website, until 1:00 pm, six flights that were to depart from Humberto Delgado Airport had been cancelled.
More cancellations are expected during the afternoon and the vast majority of flights are TAP.
As for arrivals, there are at least 21 cancelled flights.
Contacted by CNN Portugal, ANA Aeroportos said it was investigating the circumstances of the problem.
By the end of the day, close to 40 cancellations are expected.