Washington: The four-star admiral set to become the US Navy's top officer on August 1 will instead retire – an extraordinary downfall prompted by what Navy Secretary Richard Spencer called poor judgment regarding a professional relationship.

The sudden move by Admiral William Moran may add to the perception of turmoil in the Pentagon's senior ranks.

Admiral William Moran in 2018. Credit:AP

Moran had been vetted for promotion to the top uniformed position in the Navy, nominated by President Donald Trump and confirmed by the Senate in May to succeed Admiral John Richardson as chief of naval operations and as a member of the Joint Chiefs of Staff.

But Moran said in a written statement on Sunday evening, US time, that he had turned down the offer to appoint him, and requested to be allowed to retire.