Health experts are looking into reports that a coronavirus-related syndrome among children may be emerging in the UK, NHS England's national medical director has said.
Professor Stephen Powis said it was "too early to say" whether there is a link between the Kawasaki-like disease and coronavirus, but chief medical officer Prof Chris Whitty said it was "entirely plausible".
They have instructed medical experts to look into the potential link as a "matter of urgency".
It comes after the UK Paediatric Intensive Care Society (Pics) tweeted an alert it said was from NHS England which says in the "last three weeks, 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 in other regions of the UK".
NHS England confirmed it had shared the warning through its incident teams to clinical commissioning groups and hospital trusts.
The alert said the effects had been seen in children both with and without coronavirus but there was evidence that some patients had had coronavirus previously.
UK Health Secretary Matt Hancock said he was "very worried" by the reports.
Speaking at the daily Downing Street press conference, Prof Powis said: "We have become aware in the last few days of reports of severe illness in children which might be a Kawasaki-like disease.
"I have asked the national clinical director for children and young people to look into this as a matter of urgency."
He urged parents who are worried about a child who is sick and not recovering to seek medical help.
Prof Whitty added: "This is a very rare situation but I think it is entirely plausible that this is caused by this virus, at least in some cases.
"Because we know that in adults who of course have much more disease than children do, big problems are caused by an inflammatory process and this looks rather like an inflammatory process, a rather different one.
"Therefore, the possibility - it is not a definite, we need to look for other causes as well - but the possibility that there is a link is certainly plausible."
According to the alert, which was originally shared with GPs in north London, children affected display signs similar to toxic shock syndrome, a severe illness associated with infections, and have blood markers in line with severe Covid-19.
They may also have abdominal pain and symptoms of inflammation around the heart.
Symptoms include a high temperature that lasts for five days or more, often with a rash and/or swollen glands in the neck.
Children are contracting Covid-19 at the same rate as adults but are suffering less severe symptoms on the whole.