The gradual decline of Brehon law began during the Norman conquest, it continued to co-existed with an imposed legal system from the 12th century onwards. It was only in the middle seventeenth century efforts were made to completely suppress it out of existence in favour of the colonial common law system, which has remained the official legal system right up to the present day. In the late 17th century it was deemed a serious criminal offence to be found in possession of old Irish law book and often punishable by transportation to penal servitude.