It is the largest-ever closure plan by the company and will radically change the face of the bank, according to Unite.
Inpatient Children's Ward To Close
A hospital children's ward is to close to inpatients for six weeks because of staffing and recruitment issues.
NHS Lothian said the "difficult decision'' surrounding services at St John's Hospital in Livingston was taken to ensure patient safety.
The summer closure at the West Lothian hospital will take effect from Friday and will see any inpatients treated at Edinburgh's Sick Kids' Hospital instead.
Jim Crombie, chief officer of acute services at NHS Lothian, said: "We have not taken this decision lightly. The safety of children must be our top priority and all our services must be appropriately staffed.''
Last year 3,000 children were admitted to the children's ward, with 700 requiring an inpatient stay of 24 hours or more.
The health board said staffing and recruitment issues prompted the decision to implement the changes at St John's.
It said it has not been able to fill the medical and specialist nursing posts needed, despite national and international recruitment campaigns.
This has led to the service remaining "heavily reliant'' on locum doctors, as well as staff from both the Sick Kids' Hospital and St John's working extra hours.
NHS Lothian said it would "assess all possible options for a sustainable paediatric service'' for the health board area over the next six weeks.
In the meantime, the children's ward will continue to operate as an assessment and programmed investigation unit on weekdays between 8am and 8pm.
Most of the children who currently attend the unit will continue to be cared for as normal, health bosses said.
There will be no change to maternity and neonatal services at the hospital.
Mr Crombie added: "We are fully committed to maintaining and indeed expanding the role of St John's Hospital as one of Lothian's three major acute hospital sites, but if we don't make changes now to the operating hours of the children's ward, we run the risk of having to make an unplanned closure at a few hours notice, which would lead to the sudden diversion of patients.
"We have relied on our dedicated staff from St John's and across Lothian to work additional hours to provide these important services, but we cannot continue to rely indefinitely on a small number of staff working excessive hours.
"We must have a reliable and safe way of treating patients without that unacceptable risk of last-minute closure.
"It has not been possible to confidently secure enough staff over the summer and, following a detailed risk assessment, this reduction in opening hours is considered the safest option for children in West Lothian.''
Parents have been told they will not need to do anything differently, since any changes to where their children are to be treated will happen via their GP or the ambulance service.
The move comes three years after a similar closure at the hospital. In the summer of 2012, the children's ward at St John's was unable to admit patients for three weeks because of a staff shortage.
Scottish Labour's health spokeswoman Jenny Marra said the staffing shortage was "extremely worrying''.
"My first reaction is that I think they've made the right decision for patient safety because that has got to be top and foremost in the NHS managers' decision. So if there are issues with patient safety then clearly they have made the right decision.
"However, it is extremely worrying that NHS Lothian can't get enough staff to staff their key children's wards in St John's Hospital.''
She called for the Scottish Government to "get a grip'' of workforce planning within the NHS.
The SNP claims a "Hard Brexit" will cost 80,000 jobs in Scotland.
A bullet has been recovered from a tree where a man was shot near a primary school.
Scotland's jobless total rose by 11,000 in the three months to November.
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