The research agency said that the “primary cause” of the fault was URLs that were too long for its system to recognise. In cases where it encounters these long URLs, Nielsen said, its technology “intermittently does not recognise the [online] session”, leading to an estimated 22% drop in measured time spent online.

Nielsen told clients that it is “actively investigating an erroneous decline in our internet data” in the NetView service, but the effect of the glitch on the VideoCensus, MegaView Retail, MegaView Search, AdRelevance and WebRF services, and any related custom research projects, “is not yet fully understood”.

Declines have been noted in the US, but Nielsen said it has widened its probe to other countries where the affected services are available.

President of media products Steve Hasker apologised for the disruption and said that the issue would be corrected in the December data set, which will be released in January.