Health bosses are warning against using a type of heated birthing pool at home after a baby born in one was infected with Legionnaires' disease.
Public Health England (PHE) and NHS England are temporarily advising against the use of pools which have built-in heaters and recirculation pumps and can be filled up two weeks before the birth.
The alert comes after a case of the Legionnaires' lung disease was identified in a baby born in the specific type of birthing pool at home.
The child is in intensive care in hospital. It is the first reported case of Legionnaires' disease linked to a birthing pool in England, PHE said.
Samples taken from the birthing pool used confirmed the presence of legionella bacteria, which causes the disease.
Experts are carrying out tests to establish if it is the same strain as that which infected the baby.
Professor Nick Phin, PHE's head of Legionnaires' disease, said: "This is an extremely unusual situation, which we are taking very seriously.
"As a precaution, we advise that heated birthing pools, filled in advance of labour and where the temperature is then maintained by use of a heater and pump, are not used in the home setting, while we investigate further and until definitive advice on disinfection and safety is available."
NHS England issued a Patient Safety Alert to notify the healthcare system and midwives in particular of the possible risk associated with the use of the heated birthing pools at home.
The alert recommends that heated birthing pools are not used for labour or birth. A full risk assessment is being carried out in the meantime.
Heated pools from the supplier involved in the incident have been recalled, PHE said. There are around 10 firms which supply the specific pools and each have between two and 14, which they loan out.
The pools are typically delivered around a fortnight before the expected delivery date and filled from the domestic hot water supply.
The temperature is then maintained by a pump and heater until labour and delivery, with the companies recommending various disinfection regimes.
PHE said the majority of birthing pools used at home are filled from domestic hot water systems at the time of labour and these do not pose the same risk and are not included in the alert.
Prof Phin said: "We do not have concerns about purchased or hired pools that are filled from domestic hot water supplies at the onset of labour, provided that any pumps are used solely for pool emptying.
"PHE and relevant local authorities are investigating the infection control measures required for this type of birthing pool and local authorities will be working with the small number of companies who supply these heated birthing pools for use at home."
Louise Silverton, director for midwifery at the Royal College of Midwives, said: "Women planning birth at home using a traditional pool that is filled when the woman is in labour or using a fixed pool in an NHS unit are not affected by this alert and should not be concerned.
"Birthing pools in hospitals are subject to stringent infection control procedures and monitoring. Home birthing pools filled during labour come with disposable liners and are only in place for a relatively short time period, reducing opportunity for bacterial growth.
"Any women with concerns about using home birthing pools should contact their midwife or local maternity unit."
Legionnaires' disease is extremely rare in children, with only one case in youngsters aged up to nine in England between 1990 and 2011.
Although there were two cases reported in Italy and Japan several years ago, it is the first reported case of Legionnaires' disease linked to a birthing pool in England.
Patients become infected with the bacteria through inhalation of contaminated water droplets. The infection does not spread from person to person.
The disease is a severe form of pneumonia which affects around 350 to 400 people each year in England and Wales. The majority of cases involve older patients.
NHS information about the condition says that most people make a full recovery but "in some cases it can lead to further, life-threatening, problems".
Midwives and every local authority in the country are being contacted to see if they use the specific pools provided by any of the companies that supply them.
If the pools are currently out for use, councils and midwives should take steps to get them back, a PHE spokeswoman said.
The spokeswoman declined to give any further information about the sick baby, including when and where it was born and its sex.
The NHS patient safety alert described the child as "severely ill".
The company that supplied the pool involved has not been named.
Mark Fielder, Professor in Medical Microbiology at Kingston University, said: " The specific type of pool implicated can be filled in advance of labour and the temperature is maintained by the heater and pump.
"Birthing pools that are filled from domestic hot water systems at the time of labour do not appear to possess the same risk factors and are not included under the current exclusion.
"This single case does represent a rare occurrence, indeed this is the first case reported in England from a birthing pool.
"The infection is caused by a bacterium called Legionella pneumophilia which can be found commonly in water sources and at low levels it is harmless. The organism can be controlled effectively if the water is cooled to below 20?C or heated to above 60?C.
"This type of treatment coupled with general good hygiene in terms of keeping water free of impurities will help to keep the organism under control."
Legionnaires' disease can be "particularly serious" in the elderly and "immunocompromised" patients, including young infants, said Professor Nigel Brown, president of the Society for General Microbiology.
He added: " Legionella is common in the environment and one of the reasons why air conditioning systems have to be regularly serviced to remove bacterial build-up."
Carmel Bagness, midwifery and women's health adviser at the Royal College of Nursing, said: " This is a highly unusual situation and it is right that the proper precautions are taken, but women should be assured that this alert does not apply to home pools which are filled at the time of labour or fixed pools in NHS units, which do not pose the same risk.
"Any woman or partner concerned about this alert should contact their midwife or maternity unit for further information."