Health officials are investigating an unexplained rise in cases of a mysterious polio-like illness which causes paralysis of the limbs and mainly affects children.

There have been 28 cases of acute flaccid paralysis (AFP) reported in 2018 with the majority occurring since September, according to Public Health England.

Officials would usually expect fewer than 10 cases a year in England and a similar unexplained rise has also been recorded in the US this year.

PHE said the the condition mainly – but not exclusively – affects children and said the public should seek medical help “immediately” if they notice any weakness or floppiness of the limbs.

“AFP is very rare,” Dr Mary Ramsay, head of immunisations at PHE said.

“However, if you or your child develops weakness in any limb you should seek medical care immediately so that appropriate testing and care can be given.

“We are investigating potential causes and working hard to build better awareness amongst health care professionals about how to test and manage patients with AFP.”

There have been more than 120 cases of AFP reported in the US in 2018 so far, with 62 confirmed across 22 states, according to the Centre for Disease Control.

AFP usually occurs following a viral infection which affects the central nervous system. While the largely eradicated polio virus is one cause, it can also be triggered by the wider family of enteroviruses which usually only have mild cold-like symptoms.

Doctors are being told to report any suspected cases of AFP as it can be hard to diagnose without specialist laboratory support and there are many other causes of limb weakness.

Investigations usually require scans of the brain and spinal cord and may involve a lumbar puncture to draw spinal fluid to look for signs of inflammation or infection.

The US CDC has been monitoring the condition since increases were first noticed in 2014 On its website it said: "CDC estimates that less than one to two in a million children in the United States will get acute flaccid myelitis every year.