Hillary Clinton was last night forced to promise she would release further medical information after opponents accused her of trying to hide serious health issues from the American people.

In the worst setback so far to hit her chances of reaching the White House, Mrs Clinton, 68, faced questions over why she covered up a diagnosis of pneumonia for two days, reinforcing concerns about her perceived dishonesty.

Pneumonia is the eighth leading cause of death in America claiming 50,000 lives a year.

Donald Trump, her Republican opponent, immediately announced he had undergone a detailed physical examination and would release results this week.

Mrs Clinton was diagnosed with pneumonia on Friday, but the condition was only disclosed on Sunday.

It only came to light after passers-by videoed her collapsing and being lifted into a car by Secret Service agents when she left early from a September 11 memorial service in New York.

One police official described her being thrown into the car "like a side of beef," losing a shoe under the vehicle.

Mr Trump said: "They said pneumonia on Friday but she was coughing very badly a week ago. It wasn't the first time. I assume that was pneumonia also.

Something's going on...but I just hope she gets well and gets back on the trail."

US presidential historian Douglas Brinkley said: "I think it's exceedingly important that Hillary Clinton be transparent about what's going on.

"If she gets a report of pneumonia on Friday they should try to tell the public in real time. The danger for a candidate is if they seem to be hiding their health history."

Brian Fallon, Mrs Clinton's spokesman, dismissed internet conspiracy theories that she was suffering from a brain tumour or dementia, saying there was "no undisclosed condition, pneumonia is the extent of it," and that she never lost consciousness.

He also said there was no link between her current health problem and a concussion she suffered in 2012 after falling at home.

That led to a blood clot on her brain, double vision, and a six-month recovery.

Mr Fallon said the Democratic nominee would be back on the campaign trail later this week after calling off a trip to California.

Mr Trump's physical examination was carried out by Harold Bornstein, the same New York doctor who previously released a letter saying he would be "the healthiest individual ever elected to the presidency".

On Thursday Mr Trump will appear on television with celebrity physician Dr Mehmet Oz to discuss his "personal health regimen".

Mrs Clinton's fainting episode overshadowed a row about a gaffe on Friday when she dismissed half of Mr Trump's supporters as a "basket of deplorables".

She called them "racist, sexist, homophobic, xenophobic, and Islamaphobic".

Mr Trump said the "deplorables" remark was "the single biggest mistake of the political season".

He immediately spent $2 million to make it the focus of his television advertising in battleground states.

'There is no other undisclosed condition'

Hillary Clinton is suffering from no health ailments other than pneumonia, and the Democratic presidential nominee is expected to return to the campaign trail as early as mid-week, a senior aide said on Monday.

"There's no other undisclosed condition. The pneumonia is the extent of it," spokesman Brian Fallon told MSNBC a day after Mrs Clinton, 68, fell ill at a 9/11 memorial event in New York and her campaign announced she had been diagnosed with pneumonia.

"I expect that by the middle to the late of the week she'll be back out there on the campaign trail," he said, adding that the campaign will also release fresh medical records to help "further put to rest any lingering concerns" about the former secretary of state's health.

The pledge came after her Republican rival Donald Trump vowed to release detailed results of his health from a physical exam as he suggested that Mrs Clinton may have had pneumonia for longer than stated.