Hospitals should not impose blanket bans on mobile phones, regulators say.
Many hospitals ban the use of mobile phones
The Medicines and Healthcare products Regulatory Agency (MHRA) said there was confusion over whether phones ought to be banned outright.
Its experts said they should be restricted only where specialist equipment was used, as in intensive care and specialist baby units.
They said there was a small risk of interference. Bans are currently decided on individually by hospitals.
However, many NHS trusts have introduced outright bans and the British Medical Association has called on doctors to be allowed to use phones, but not the public.
The MHRA said it was introducing the guidance after a series of enquiries from NHS staff and patients.
It said there had been 10 reports in the last decade of mobiles interfering with infusion pumps, used to administer fluids.
Professor Kent Woods, the chief executive of the MHRA, said: "The agency recommends that mobile phones are not used in critical care areas or where patients are attached to complex devices as any effect on such equipment could be detrimental to patient care."
The MHRA said this would include many electronic medical devices such as dialysis machines.
But Maria Nyberg, policy manager at the NHS Confederation, which represents NHS trusts, said it should be left to individual trusts to decide what to do.
She said as well as considering the MHRA guidance, there were other factors to take into account.
"Trusts will also consider potential noise disturbance caused by the use of mobile phones in hospitals.
"Mobile phones can often be intrusive and technological advances mean that increasingly phones have cameras and recording devices.
"Patients have the right to a peaceful environment and to be treated with dignity and respect at all times during their encounter with NHS services."