NHS doctors have been warned to look out for a rare but dangerous reaction in children that may be linked to Covid-19 infection.

An urgent alert sent out to GPs said that intensive care departments in London and other parts of the United Kingdom have been treating severely ill children with unusual symptoms which include “multi-system inflammation” with flu-like symptoms.

Some, but not all, tested positive for coronavirus.

It is unclear how many children have been diagnosed with the new syndrome, although the number is expected to be low.

The alert, which was issued by NHS England, said that there was a growing concern that a coronavirus-related inflammatory syndrome is immerging in children in the UK or that there may be another, as yet unidentified, infection linked to these cases.

Dr Nazima Pathen, a consultant in Paediatric Intensive Care, in Cambridge, said that colleagues in Spain and Italy had been reporting similar cases: “Some of the children have presented with a septic shock type illness and rashes – the kind of presentation we would expect to see in toxic shock syndrome and Kawasaki disease (which affects blood vessels and the heart).

“Overall, children seem to be more resilient to serious lung infection following exposure to coronavirus, and the numbers admitted into Intensive Care Units are relatively low.

NHS England said that it knew of fewer than 20 such cases in the country where an association has been noted by clinicians.

A spokesperson said that investigations will continue, but no link has yet been established.

The Royal College of Paediatrics and Child Health (RCPCH) said parents should be reassured by this, but if they are concerned about their children’s health for any reason, they should seek help from a health professional.