Donald Trump's long-time doctor has revealed he wrote a letter endorsing the candidate's health in just five minutes while a limo was waiting outside - but refused to back away from the glowing description he gave The Donald.

Dr Harold Bornstein, a gastroenterologist based in the Manhattan's Upper East Side, described Trump as the picture of health on Friday.

'His health is excellent, especially his mental health,' the doctor told NBC News, doubling down on a letter he wrote last December about Trump's well-being.

'If elected, Mr. Trump, I can state unequivocally, will be the healthiest individual ever elected to the presidency,' Bornstein wrote in his medical endorsement of the Republican nominee last year.

Donald Trump's long-time doctor Harold Bornstein (pictured) has revealed he wrote a letter endorsing the candidate's health in just five minutes while his patient's limo waited outside

When the doctor, who has had Trump as a patient since 1980, was asked by NBC about the bombastic statement, he said: 'I like that sentence to be quite honest with you.'

'I think I picked up his kind of language and then just interpreted it to my own,' he told the network.

In his December letter, Borstein also said Trump had lost 15 pounds over the past year, has 'suffered no form of cancer', and has 'extraordinary' strength and stamina.

The health of both nominees has become a big issue in recent weeks after intense scrutiny of Hillary Clinton's well-being spread from right-wing websites to allies of Trump and people within his campaign.

Dr Harold Bornstein (pictured), a gastroenterologist based in the Manhattan's Upper East Side, described Trump as the picture of health on Friday

'His health is excellent, especially his mental health,' the doctor said, doubling down on a letter he wrote last December about Trump's well-being

'If elected, Mr. Trump, I can state unequivocally, will be the healthiest individual ever elected to the presidency," Bornstein wrote in December

Some critics of the Democratic nominee have highlighted old footage to suggest she had a seizure on camera, while former New York City Mayor Rudy Giuliani suggested the media is covering up Clinton's alleged health concerns.

'All you have to do is go online,' Giuliani said during an appearance on Fox News.

'Go online and put down "Hillary Clinton's illness." Take a look at the videos for yourself.'

Giuliani was referring to a number of right-wing conspiratorial videos that suggest the 68-year-old Clinton is plagued by a number of health problems.

'I think I picked up his kind of language and then just interpreted it to my own,' Dr Harold Bornstein said about his letter about Trump's health

In his December letter, Borstein also said Trump had lost 15 pounds over the past year, has 'suffered no form of cancer', and has 'extraordinary' strength and stamina

Bornstein wrote this letter for Trump in December last year, before the businessman had secured the Republican nomination

Clinton's campaign began vehemently pushing back against online rumors that she is ill, using comments made by Trump to get on the topic.

Trump had said during a national security speech that Clinton lacked the 'mental and physical stamina to take on ISIS.'

Then, during a town hall with Fox News Channel's Sean Hannity, Trump said Clinton 'doesn't really do that much. She'll give a speech on a teleprompter, and then she'll disappear.'

'While it is dismaying to see the Republican nominee for president push deranged conspiracy theories in a foreign policy speech, it's no longer surprising,' Clinton's communications director Jennifer Palmieri said in a statement.

The health of both candidates has been questioned, after scrutiny of Hillary Clinton's well-being spread from right-wing websites to allies of Trump and people within his campaign

Clinton's campaign began vehemently pushing back against online rumors that she is ill

'Donald Trump is simply parroting lies based on fabricated documents promoted by Roger Stone and his right wing allies.'

Clinton's team also released a new statement from her physician, Dr Lisa Bardack, who signed off on the Democrat's health in the beginning months of her campaign last year.

'As Secretary Clinton's long time physician, I released a medical statement during the campaign indicating that she is in excellent health,' Bardack said.