The issue of Hillary Clinton’s health has moved front and centre to the presidential election campaign after her doctor said she was being treated for pneumonia - hours after she stumbled and felt unwell at a 9/11 memorial service.

Dr Lisa Bardack said Ms Clinton was “recovering nicely” after attending the event in lower Manhattan on Sunday morning. Ms Clinton was seen on video struggling to stay on her feet, and had to be helped by Secret Service staff as she got into a vehicle to leave the event early.

Ms Clinton, 68, was due to fly to San Francisco and Los Angeles on Monday, but a campaign official said she had cancelled the two-day California trip that was to include a key speech on the economy and fundraising events.

Ms Clinton appeared to stumble as she got into her vehicle (YouTube)

The huge injection of drama into the campaign came as Ms Clinton’s lead over rival Donald Trump had been slipping, both nationally and in some of the crucial swing states that will likely decide the race. Her campaign has also been fighting allegations from Mr Trump and his supporters that she is seriously unwell and trying to conceal something from the public.

In pictures: Hillary Clinton's 2016 presidential bid Show all 30 1 /30 In pictures: Hillary Clinton's 2016 presidential bid In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 11/09/2016: Hillary Clinton waves to the press as she leaves her daughter's apartment building after resting in New York. Clinton departed from a remembrance ceremony on the 15th anniversary of the 9/11 attacks after feeling "overheated," but was later doing "much better," her campaign said AFP/Getty Images In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 25/08/2016: Hillary Clinton speaks at a campaign event in Reno, Nevada AFP In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 27/07/2016: US President Barack Obama and Democratic presidential nominee Hillary Clinton embrace on stage during Day 3 of the Democratic National Convention at the Wells Fargo Center in Philadelphia, Pennsylvania AFP/Getty Images In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 11/07/2016: Bernie Sanders officially endorsed Hillary Clinton, saying his progressive vision for ‘a transformed America’ would be ’best served by the defeat of Donald Trump’ Reuters In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 26/06/2016: New York Governor Andrew Cuomo, former Secretary of State and Democratic Presidential candidate Hillary Clinton, Al Sharpton and New York mayor Bill de Blasio march during the New York City Pride March in New York City Getty Images In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 7/06/2016: Democratic presidential candidate Hillary Clinton gestures as she greets supporters at a presidential primary election night rally in New York AP In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 26/05/2016: A supporter of democratic presidential candidate former Secretary of State Hillary Clinton holds up a shirt during a campaign event in San Francisco, California Getty Images In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 26/04/2016: Hillary Clinton embraces her husband, former President Bill Clinton, at a primary night campaign event in Philadelphia, Pennsylvania. Early results indicated Clinton would win Pennsylvania's presidential primary Getty Images In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 8/03/2016: Hillary Clinton and Bernie Sanders during of the CNN Democratic Presidential candidates' debate, at the Whiting Auditorium in Flint, Michigan EPA In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 3/03/2016: Singer Katy Perry raises arms with U.S. Democratic presidential candidate Hillary Clinton at the end of the Hillary Victory Fund "I'm With Her" benefit concert at Radio City Music Hall in the Manhattan borough of New York City REUTERS In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 28/02/2016: Hillary Clinton delivered a victory speech in Columbia AP In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 16/02/2016: Hillary Clinton speaks at a rally at Truckee Meadows Community College in Reno, Nevada AP In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 9/02/2016 (L-R) Priyadharshini Shanthakumar, Hattie Hughes and Claudia Blair with Hillary Clinton campaigning in Derry, New Hampshire Hattie Hughes In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 30/01/2016: Hillary Clinton is introduced by her daughter Chelsea while the two campaigned together at a 'Get Out The Vote" caucus' event at the Carrollton Inn in Carroll, Iowa Getty Images In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 14/12/2015: Hillary Clinton speaks during the National Immigrant Integration Conference in New York City. During the speech Clinton announced her immigration proposals if elected president Getty Images In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 9/11/2015: Hillary Clinton speaks on stage during a rally after filing paperwork for the New Hampshire primary at the State House in Concord, New Hampshire. Each candidate must file paperwork to be on the New Hampshire primary ballot, which will be held on 9 February 2016 Getty Images In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 22/10/2015: Hillary Clinton avoided major damage to her presidential campaign during a nearly 11-hour congressional hearing dominated by Republican criticism of her response to the Benghazi attacks AFP In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 9/09/2015: Hillary Clinton takes part in a discussion after speaking about the Iran nuclear deal at the Brookings Institution in Washington. Clinton expressed firm support for the nuclear accord with Iran, calling it flawed but still strong. Clinton added that the agreement must be strictly enforced and said that if elected president next year, she would not hesitate to use military force if Iran fails to live up to its word and tries to develop a bomb AFP/Getty Images In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 8/09/2015: Hillary Clinton does the "Nae Nae" dance move with DJ Stephen "Twitch" Boss (L) and television host Ellen DeGeneres (R) during a taping of "The Ellen DeGeneres show" in New York In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 26/08/2015: Hillary Clinton greets guests during a campaign event at Tabor Home Vineyards and Winery in Baldwin, Iowa. A recent poll has Clinton leading all other Democratic contenders in Iowa by about 30 percentage points Getty Images In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 13/07/2015: Hillary Clinton poses for a selfie with a student outside the New School after speaking to outline economic vision in New York. Clinton promised to raise incomes of hardworking Americans and rein in excesses on Wall Street in the first major economic policy address of her 2016 campaign AFP/Getty Images In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 15/06/2015: Hillary Clinton speaks during a forum on early childhood education at the YMCA of Strafford County in Rochester, New Hampshire. Clinton held a formal launch event in New York City, and is hitting the first in the nation primary state for the third time since announcing her candidacy in April Getty Images In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 22/05/2015: Hillary Clinton tours the Smuttynose Brewery in Hampton, New Hampshire Getty Images In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 23/04/15: Hillary Clinton addresses the Women in the World Conference in New York City Getty In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 22/04/15: Hillary Clinton waves to college students as she leaves after speaking during the Hillary Rodham Clinton Award for Advancing Women in Peace and Security at Georgetown University in Washington AFP/Getty In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 21/04/15: Hillary Clinton meets with audience members after speaking with students and faculty of New Hampshire Technical Institute in Concord, New Hampshire. The conversation revolved around higher education, manufacturing and women in the work force Getty In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 13/04/15: “Loaded the van & set off for IA. Met a great family when we stopped this afternoon. Many more to come.” A picture of one family Hillary Clinton met on her road trip, which she tweeted In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 12/04/15: The building that will house Hillary Clinton's campaign headquarters, on the corner of Pierrepont and Clinton Streets in historic Brooklyn Heights, NY Corbis In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 12/04/15: Some expect Hillary Clinton’s campaign to cost more than Barack Obama’s $1bn in 2012 EPA In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 12/04/15: Hillary Clinton formally declares in a video that she will seek the Democratic Party’s presidential nomination for 2016, and says she wants to bring the benefits of the economic recovery to all Americans AP

On Sunday evening, US media were quick to ask why, given that Ms Clinton was apparently diagnosed with pneumonia on Friday, her campaign failed to disclose this until it was effectively forced to do so.

Mr Trump, who also attended the same event as Ms Clinton, was conspicuously silent about the development, his only comment on the 15th anniversary of the attacks on New York and Washington, being a statement calling on people to pray for those lost in the Al-Qaeda assault that left almost 3,000 people dead.

Asked by a reporter about the incident, Mr Trump said: “I don't know anything.”

News that Ms Clinton had stumbled and left early from the event was first reported by Fox News, one of the channels which has been providing a platform for opponents of the candidate to make allegations about her health.

It said she had left early after standing for 90 minutes in the humidity at Ground Zero. She walked away from the ceremony but was seen to trip and stumble as she climbed into a waiting van.

For an hour-and-a-half, the media pool traveling with her had no idea where she was, and the candidate’s campaign declined to provide details.

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It then transpired that Ms Clinton had been driven to her daughter’s apartment in the Flatiron neighbourhood of the city. Around noon, she emerged, walking by herself, and waving to the media and waiting crowds.

She briefly posed for a photograph with a young girl. To a shouted question from a reporter as to whether she felt better, she said: “Yes, thank you.”

“I’m feeling great,” Ms Clinton said. “It’s a beautiful day in New York.”

Her campaign spokesman said that Ms Clinton had left the event because she had overheated.

“During the ceremony, she felt overheated so departed to go to her daughter's apartment, and is feeling much better,” spokesman Nick Merrill said in a statement.

Clinton leaves apartment after 'episode' at 9/11 memorial

Ms Clinton was then taken to her home in upstate New York where the media gathered in a nearby hotel. It was then that Ms Clinton’s doctor issued the details about her diagnosis.

Ms Bardack said that Ms Clinton had had an allergy-related cough, and that during a follow-up examination Friday, the candidate was diagnosed with pneumonia, put on antibiotics, advised to rest and modify her schedule.

“I have just examined her and she is now rehydrated and recovering nicely,” she said, according to the Associated Press.

It remains unclear how serious Ms Clinton’s condition is, and how much it will impact her campaign. What is clear is that the episode looked serious, and was being used by her critics as evidence of her unsuitability for the White House.

With many polls showing Ms Clinton’s lead over Mr Trump narrowing, she created controversy last Friday by claiming that half of Mr Trump’s supporters were a “basket of deplorables” - a comment that drew sharp criticism from Republicans.

Ms Clinton later said she regretted applying that description to “half" of Mr Trump’s backers, but stuck by her assertion that the Republican nominee has given a platform to “hateful views and voices”.

Mr Trump and Ms Clinton are among the oldest candidates to contest for the White House in US political history. Mr Trump has sought to question Ms Clinton’s physical and mental ability to do the job. Ms Clinton and her supporters have repeatedly questioned the New York tycoon’s temperament.

Ms Clinton suffered concussion in December 2012 after fainting. Her doctor attributed that episode to a stomach virus and dehydration.

Ms Clinton’s doctor reported she is fully recovered from the concussion, which led to temporary double vision and discovery of a blood clot in a vein in the space between her brain and skull.

Ms Clinton also has experienced deep vein thrombosis, a clot usually in the leg, and takes the blood thinner Coumadin to prevent new clots. In July 2015, Ms Clinton issued a detailed two-page letter from her physician that included the concussion.

Mr Trump’s personal physician has said the Republican presidential nominee is in excellent health both physically and mentally. But the former reality television star has refused to release his own health records.

Dr Harold Bornstein’s report last December remains the only medical information released so far by the Mr Trump campaign.