By the time of the election in November 1968, LBJ had evidence Nixon had sabotaged the Vietnam war peace talks - or, as he put it, that Nixon was guilty of treason and had "blood on his hands".

The BBC's former Washington correspondent Charles Wheeler learned of this in 1994 and conducted a series of interviews with key Johnson staff, such as defence secretary Clark Clifford, and national security adviser Walt Rostow.

But by the time the tapes were declassified in 2008 all the main protagonists had died, including Wheeler.

Now, for the first time, the whole story can be told.

It begins in the summer of 1968. Nixon feared a breakthrough at the Paris Peace talks designed to find a negotiated settlement to the Vietnam war, and he knew this would derail his campaign….