An urgent alert has been issued to doctors about a new coronavirus-related condition seen emerging in children.

The warning says there has been an “apparent rise in the number of children of all ages presenting with a multi-system inflammatory state requiring intensive care across London and also other regions of the UK” over the last three weeks.

The NHS England alert, shared by the Paediatric Intensive Care Society (PICS) on Sunday evening, adds: “There is a growing concern that [a Covid-19-related] inflammatory syndrome is emerging in children in the UK, or that there may be another, as yet unidentified, infectious pathogen associated with these cases.”

It is not known how many children have been affected, however, PICS said it was aware of only “a small number of children nationally who appear to fit the clinical picture described in the alert”.

The illness has been seen in children who have tested positive for the novel coronavirus, as well as those who have not had the disease.

The condition has the characteristics of severe Covid-19 and also shares certain features of toxic shock syndrome and atypical Kawasaki disease – which causes the blood vessels to become inflamed and swollen – according to the alert.

Abdominal pain and gastrointestinal symptoms “have been a common feature” of the illness, as has cardiac inflammation.

Doctors have been asked to urgently refer any patients showing signs of these.

PICS said in a statement to medical professionals on Monday: “Over the weekend, PICS members received an email alert from NHS England highlighting a small rise in the number of cases of critically ill children presenting with an unusual clinical picture.

“Many of these children had tested positive for Covid-19, while some had not.

“It is important to highlight that, both in the UK and in other countries, there have still been very few cases of critically unwell children with Covid-19 admitted to paediatric intensive care units.

“However, an early case report relating to Covid-19 presenting as Kawasaki syndrome has been published recently, and PICS is aware of a small number of children nationally who appear to fit the clinical picture described in the NHS England alert.”

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Professor Russell Viner, president of the Royal College of Paediatrics and Child Health, has sought to reassure parents that children are unlikely to become seriously ill with Covid-19.

He said: ”We already know that a very small number of children can become severely ill with Covid-19 but this is very rare – evidence from throughout the world shows us that children appear to be the part of the population least affected by this infection.

“New diseases may present in ways that surprise us, and clinicians need to be made aware of any emerging evidence of particular symptoms or of underlying conditions which could make a patient more vulnerable to the virus.”

Professor Adilia Warris, paediatric infectious diseases specialist at the University of Exeter, added: “Children have so far accounted for between 1 per cent and 5 per cent of diagnosed Covid-19 cases, have often milder disease than adults and deaths have been extremely rare.

“The largest series of paediatric cases originates from China, with 5.6 per cent of 2,143 children showing severe disease (e.g. needing oxygen), and 0.6 per cent needing intensive care treatment.

“So far, children with underlying compromised immune systems or using immunosuppressive treatments (corticosteroids) are not at increased risk for developing severe Covid-19 disease.”

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Meanwhile, Dr James Gill, honorary clinical lecturer at Warwick Medical School and locum GP, said it was too soon to say whether there was any link between the “multi-system inflammatory state” described in the NHS alert and Covid-19.

He said: “Whilst it is easy to draw conclusions suggestive of a connection with Covid-19, there is not sufficient evidence to substantiate that the signal has any clinical significance.

“The PHE surveillance guidance on children with regard to Covid-19 does not currently mention issues with a multi-system inflammatory response.

“In many ways, there is no change here. Clinically health professionals have a heads-up about possible presentations, but will not change their current high level of caution.”

Boris Johnson announced the UK coronavirus lockdown cannot yet be eased as he returned to work on Monday after recovering from the disease for three weeks.

The prime minister said there were signs the country was “turning the tide” against the virus but warned the public that it was too early to lift the restrictions.