The navicert, short for navigation certificate,[1] also known as a "Letter of Assurance", was a form of commercial passport issued to allow ships to pass through blockades without inspection.
Navicerts were introduced at the suggestion of US Consul General in London Robert Peet Skinner and first issued in March 1916, as a method of reducing the inconvenience of the blockade to neutral trade.
Vessels whose entire cargo is covered by navicerts will be subjected to the minimum of delay.Navicerts were approached legally as the British authorities voluntarily refraining from their rights of visit and search.
This meant that these ships whose entire cargoes were navicerted could avoid delays and uncertainty from being stopped and searched, or diverted to a British port for inspection.
In World War II the system was quickly begun in December 1939 under the Ministry of Economic Warfare, and became compulsory by July 1940, with 5000–7000 applications made per month in 1941.