Singaporean police have arrested a British author as part of a criminal defamation investigation related to his book on the city-state's death penalty policy.

Alan Shadrake, 75, was in Singapore to promote the book, Once a Jolly Hangman: Singapore Justice in the Dock.

Police said they arrested him on the basis of a complaint by the Government's Media Development Authority and were investigating him for other offences.

The Attorney-General's Office is also pursuing contempt of court charges because statements in the book allegedly impugn the impartiality and independence of the judiciary.