The recordings are the last of a total of 3,000 hours of tape released by the National Archives and Records Administration. The newly released tapes reveal two future US presidents (Reagan & H.W. Bush) called Richard Nixon in support after he gave a speech on the Watergate scandal amid a staff exodus.

Another 700 hours remain restricted by national security and privacy concerns, but the archive says they will now be reviewed in order to see what can be released.

The tapes cover the time period between April 9 and July, 12 1973, the day before the existence of Nixon's secret recording system was made public to a Senate panel probing the scandal.

It includes the day three senior White House officials resigned over the affair and another was fired.

Nixon remains the only US president to resign.