US President Donald Trump has claimed the NHS is "going broke and not working" as he targeted rival Democrats pushing for a universal health system.
In his tweet he also said "thousands of people are marching" about it.
This was believed to be a reference to a Save the NHS march on Downing Street on Saturday demanding more funding for the health service.
Asked about his remarks, Downing Street said Theresa May was "proud" of the UK's system.
Her spokesman said the NHS had recently been ranked as the world's best healthcare system and that Health Secretary Jeremy Hunt, who has hit back at the US president on Twitter, "speaks for the government".
In the UK the NHS is funded out of general taxation, so people do not have to pay when they get treatment. In the US, when people get treatment they have to pay, most often through health insurance providers.
President Trump's tweet came after ex-UKIP leader Nigel Farage appeared on Fox And Friends, one of the president's favourite shows, talking about the weekend march.
"Dems want to greatly raise taxes for really bad and non-personal medical care," Mr Trump tweeted.
In response, Mr Hunt said that while he "disagreed with claims made on that march", no-one wanted "to live in a system where 28 million people have no cover".
He added: "NHS may have challenges but I'm proud to be from the country that invented universal coverage - where all get care no matter the size of their bank balance."
Labour leader Jeremy Corbyn - the leader of the UK opposition - also hit back, saying: "People were marching because we love our NHS and hate what the Tories are doing to it."
He added: "Healthcare is a human right."
NHS funding has been hotly debated in the UK as hospitals struggle to cope with the pressure on resources.
A panel set up by the Liberal Democrats - the fourth largest party in the Commons - has called for a ring-fenced tax to fund the service, saying an extra £4bn was needed for next year and an additional £2.5bn for both 2019 and 2020.
Lib Dem leader Sir Vince Cable urged the US president to read the study, "to find out how to fund a universal healthcare system".