Hillary Clinton was on Sunday night facing serious questions about her health as she cancelled a campaign trip to California after admitting she was suffering from pneumonia.

In what could be a major turning point in the bitterly contested presidential campaign, Mrs Clinton appeared to faint as she departed early from a ceremony honouring victims of the September 11 attacks at Ground Zero in New York City.

At first her campaign team claimed that Mrs Clinton had "overheated". But her doctor was later forced to admit that she had been diagnosed on Friday with a much more serious illness.