Charlie Gard's parents have abandoned a legal fight over treatment for their terminally-ill baby son after concluding that he had deteriorated to the "point of no return".
But Chris Gard and Connie Yates still think that 11-month-old Charlie might have been saved if experimental therapy had been tried sooner.
Ms Yates told a High Court judge who has overseen the dispute that if Charlie had been treated at the start of the year he would have had the potential to be a "normal, healthy little boy".
She said time had been "wasted".
Doctors at Great Ormond Street Hospital in London did not agree.
Lawyers representing the hospital told Mr Justice Francis that the "clinical picture" six months ago had shown irreversible damage to Charlie's brain.
They said the "unstoppable effects" of Charlie's rare illness had become plainer as weeks passed.
A barrister representing Charlie's parents on Monday drew the five-month legal battle to a close at a hearing in the Family Division of the High Court in London.
Grant Armstrong told the judge that the couple had decided to stop pushing for Charlie to be allowed to undergo a therapy trial overseen by a specialist in New York.
He said they had made the "most painful of decisions" after reviewing new scan results.
Ms Yates outlined her thoughts in a statement. She read it from the witness box with Mr Gard at her side.
The couple wept as Ms Yates told a court packed with lawyers, relatives, supporters and journalists: "We are now going to spend our last precious moments with our son Charlie, who unfortunately won't make his first birthday in just under two weeks' time."
Ms Yates said the latest scan results had forced a change of heart.
"We are truly devastated to say that following the most recent MRI scan of Charlie's muscles ... we have decided that it's no longer in Charlie's best interests to pursue treatment and we will let our son go and be with the angels," she said.
"Our son has an extremely rare disease for which there is no accepted cure, but that does not mean that this treatment would not have worked, and it certainly does not mean that this shouldn't have been tried."
She added: "We have been asking for this short trial for the past eight months. Charlie did have a real chance of getting better if only therapy was started sooner. It was never false hope, as confirmed by many experts.
"Now we will never know."
Ms Yates said: "Our poor boy has been left to just lie in hospital for months without any treatment whilst lengthy court battles have been fought.
"We have been told time and time again that Charlie has a 'progressive disease' but rather than allow treatment for him with a medication that was widely accepted to have no side effects, Charlie has been left with his illness to deteriorate, sadly, to the point of no return."
Barrister Katie Gollop QC, who led Great Ormond Street's legal team, told the judge: "Charlie's parents believe that his brain was not damaged, that it was normal on the MRI scan in January and that treatment could have been effective at that time."
But she added: "There remains no agreement on these issues. Great Ormond Street Hospital treats patients and not scans. All aspects of the clinical picture and all of Charlie's observations indicated that his brain was irreversibly damaged and that (the therapy) was futile."
She went on: "As the weeks have passed, the unstoppable effects of Charlie's aggressive, progressive, depletive disorder have become plainer to see."
A barrister representing a guardian, who cannot be named but was appointed by the judge to independently represent Charlie's interests, said the issue had always been whether the therapy was in "Charlie's best interests".
"The guardian approached the evidence in respect of the (therapy) with an open mind," said Victoria Butler-Cole.
"In April 2017, the unanimous evidence of the numerous medical experts ... was that Charlie had largely irreversible brain damage and that meaningful improvement in his brain function following (therapy) was highly unlikely."
She added: "The guardian hopes that a legacy of these very distressing and emotional proceedings will be recognition that better communication is required at an early stage, not just between doctors and patients, but also between doctors offering novel treatments and those caring for the patient.
"If novel therapies are to be offered, it appears to the guardian to be imperative that those offering to provide them are fully aware of the clinical condition and medical history of the particular patient and they have had extensive discussion with the treating team, so that offers are made on an informed basis and without setting up false hope and expectations."
The court fight had started in March.
Mr Gard and Ms Yates, who are in their 30s and come from Bedfont, west London, had asked Mr Justice Francis to rule that Charlie should be allowed to undergo a therapy trial in New York.
Doctors at Great Ormond Street said the therapy was experimental and would not help.
They said life-support treatment should stop.
Mr Justice Francis in April ruled in favour of Great Ormond Street and said Charlie should be allowed to die with dignity.
He said Great Ormond Street doctors had considered the experimental treatment on offer in America but decided that it would not help.
The judge said the case had never been ''about money''.
Charlie's parents subsequently failed to overturn his ruling in the High Court, Court of Appeal and Supreme Court in London.
They also failed to persuade European Court of Human Rights judges to intervene.
But the couple had recently returned to court saying they had new evidence and they asked Mr Justice Francis to change his mind.
Mr Justice Francis had told the couple that he would not re-run the case but would consider any ''new material''.