April 9 (Reuters) - TransCanada Corp said it received authorization from the Pipeline and Hazardous Materials Safety Administration on Saturday evening to restart the 590,000 barrel per day Keystone crude pipeline at reduced pressure.

The Canadian pipeline company said that regulator PHMSA has approved a return to service plan for a controlled start. It was not immediately clear when and if the pipeline had restarted.

The pipeline, which delivers light and heavy crude from Hardisty, Alberta, to Cushing, Oklahoma, and Illinois, was shut last Saturday after a potential leak was discovered in South Dakota.

(Reporting by Catherine Ngai in New York; Editing by Matthew Lewis)