Barack Obama is reportedly preparing to endorse Hillary Clinton as early as this week, and will campaign aggressively to stop Donald Trump from succeeding him as president.

The president has so far remained on the periphery of the primary election contests, as Mrs Clinton has struggled to see off a challenge from Bernie Sanders and Mr Trump has claimed the Republican nomination.

With his former secretary of state set to clinch the Democratic nomination on Tuesday, there are talks underway for how and when Mr Obama will officially make his endorsement, the New York Times reported.

He has indicated that he sees Mrs Clinton as the best guardian of his legacy, and will relish the opportunity to challenge Mr Trump more openly.