A 13-year-old British boy has died days after testing positive for COVID-19, hospital officials and his family said on Tuesday, with relatives saying he had no underlying illnesses.

The boy, who died Monday at King's College Hospital in London, is believed to be Britain's youngest confirmed death in the coronavirus pandemic. A 12-year-old girl, whose death was confirmed earlier on Tuesday in Belgium, is thought to be Europe's youngest victim.

The boy's family said Ismail Mohamed Abdulwahab "started showing symptoms and had difficulties breathing" before he was admitted to hospital.

"He was put on a ventilator and then put into an induced coma but sadly died yesterday morning," the family said through a family friend, Mark Stephenson, adding: "We are beyond devastated."

Nathalie MacDermott, a lecturer at King's College, said: "While we know it is much less likely for children to suffer severe COVID-19 infection than older adults, this case highlights the importance of us all taking the precautions we can to reduce the spread of infection in the UK and worldwide."

She urged research into deaths outside the groups expected to succumb to infection as it "may indicate an underlying genetic susceptibility."

On Tuesday, Britain announced 381 deaths from COVID-19 in the past 24 hours, the highest figure in the country since the start of the pandemic, bringing the death toll to 1,789.