Patrick Foster was brought in for questioning, The Guardian reported. Metropolitan Police said only that the arrest involved the "identification of a previously anonymous blogger in 2009."
The Guardian said the blogger was believed to be Richard Horton, a constable in Lancashire who was forced to shut down his NightJack blog after The Times in London identified him as the blogger. NightJack won the Orwell Prize for its reporting on the life of a police constable, but Horton got a reprimand.
Foster, who began working for The Times as a trainee, left in 2011. He has done freelance work for the Guardian and The Daily Telegraph.
Times executives testified about the NightJack affair before the Leveson Commission, which is investigating newspaper conduct. Horton has filed a lawsuit against the newspaper.
Notable deaths of 2014 [PHOTOS]
Syrian Al Qaida group executes Lebanese soldier