Hillary Clinton’s campaign has admitted it could have done better in its handling of news that the Democratic candidate was suffering from pneumonia.

Ms Clinton was filmed almost buckling at the knees after leaving early from a 9/11 memorial event in New York on Sunday.

It later emerged that she had been diagnosed with pneumonia on Friday, after suffering a number of coughing fits. Details of her illness were not initially made public.

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The episode has sparked fresh concerns that the health of the 68-year-old is becoming a major campaign issue and reinforces critics' claims that the Clintons and their campaigns lack transparency.

Jennifer Palmieri, the campaign’s communications director, responded to criticism from David Axelrod, President Barack Obama’s former chief campaign strategist, that Ms Clinton took privacy too far.

The accusation follows her difficulties in explaining why she used a private email server during her time as Secretary of State and lingering questions about the finances of the Clinton Foundation.

The spat was immediately spotted by Mr Trump’s campaign, who weighed in on Twitter.

Ms Clinton has cancelled a two-day swing through California and is instead resting at her home in upstate New York.

Her health has been a frequent target of conspiracy theorists throughout the campaign.

Mr Trump used a telephone interview on Monday morning to wish her a speedy recovery. He added that he had recently had a physical and would be releasing the results.

The health problem marks the latest blow for Ms Clinton at a time when Mr Trump has eaten into much of her lead in national opinion polls, He is also challenging her in many battleground states where the election is likely to be decided.