When John McCain took a swipe at rich Americans avoiding the Vietnam war draft with a medical diagnosis of “bone spurs” his target was obvious.

One of Donald Trump’s five deferments was the result of a doctor’s letter stating that he suffered from bone spurs in his feet.

The veteran senator may not have named the president as his target, but his comments suggest they were a long delayed retort to Mr Trump’s mockery of Mr McCain’s war record.

In an interview broadcast by C-Span on Sunday to mark the 50th anniversary of being shot down over North Vietnam, followed by spending the next five and a half years as a prisoner of war, Mr McCain launched his latest round of attacks on the president.

“One aspect of the conflict, by the way, that I will never ever countenance is that we drafted the lowest-income level of America, and the highest-income level found a doctor that would say that they had a bone spur,” said the former Navy pilot.