Because Prince William is not yet heir to the throne, his wedding to Kate Middleton is not classed as a ‘state occasion’ – and the couple feel under no pressure to fill the 2,000-strong guest list with heads of state, the Mail understands…

But the decision to exclude the American premier and his wife Michelle from the celebrations marks a break from tradition.

Prince Charles and Princess Diana invited then-American president Ronald Reagan and his wife Nancy to their 1981 ceremony – though she came alone because the president was too ill to travel following an assassination attempt on him.

A senior courtier said: ‘It is certainly not the case that all foreign heads of state will be excluded. The guest list is still being drawn up and could change, but as things stand it’s right (to say Mr Obama will not be invited).