Archie Battersbee, who is on life support, is set to have treatment withdrawn at 2pm on Monday unless the government complies with an injunction from the United Nations’ High Commissioner for Human Rights.
Barts Health NHS Trust, which is treating the 12-year-old, said in a letter to his parents that "no supplemental oxygen will be given" after the endotracheal tube of the mechanical ventilator is removed.
Archie has relied on the machine to breathe since being admitted to hospital on 7 April after being found unconscious at home by his mother.
"The time it takes for the heart to stop beating is often a matter of minutes, but in some cases, this can take longer," the letter continued.
"A doctor will assess Archie regularly to confirm that the heart has stopped beating but with consideration of the family's need not to have too much intrusion at such a difficult time."
Archie's parents Hollie Dance and Paul Battersbee will be told on Monday morning how the withdrawal process is to be performed, with the aim to "preserve Archie's dignity", the letter read.
It went on: "You or any of the family may wish to lie on Archie's bed with him or have him in your arms, if that should be practically possible."
A High Court judge had ruled that ending treatment is in Archie's best interests, after reviewing evidence from clinicians and said the boy's prognosis was "bleak".
The family says doctors should give Archie a chance to recover and have made an application to the UN Committee on the Rights of Persons with Disabilities, citing Articles 10 and 12 of the Convention (UNCRPD) which call on nations to ensure the right to life and equal rights for disabled people.
In a letter to Ms Dance and her barrister Mr Bruno Quintavalle, the committee writes it has "requested the state party [the UK] to refrain from withdrawing life-preserving medical treatment, including medical ventilation and artificial nutrition and hydration from the alleged victim while the case is under consideration by the committee".
On Saturday the government told Sky News it has received correspondence from the UN which it is carefully considering.
A Department of Health and Social Care spokesperson said: "We recognise this is an exceptionally difficult time for Archie Battersbee's family and our thoughts are with them.
"We have received the letter and will respond in due course."
Ms Dance told Sky News she is waiting anxiously for the British government to comply with the UN request.
Barts Health NHS Trust said delaying the start of palliative care would "not be appropriate without an order of the court."
The family said the assertions were misleading, adding: "We as a family are very disappointed that the Trust's management has chosen to hide behind euphemisms and to mislead the public.
"It is hard to see any reason for that behaviour except knowing that what they are doing is cruel and wrong."