Charlie Gard's mother Connie Yates paid tribute to her "warrior" baby son after abandoning a legal fight for treatment.
Ms Yates stood with Charlie's father Chris Gard to praise Charlie.
The couple wept as she told the High Court: "So sorry that we couldn't save you."
"Our son is an absolute warrior and we could not be prouder of him and we will miss him terribly," Ms Yates told the court as she and her husband announced they were giving up their fight to keep their son alive.
"One little boy has brought the world together and whatever people's opinions are, no one can deny the impact our beautiful son has had on the world and his legacy will never ever die.
"Charlie has had a greater impact on and touched more people in this world in his 11 months than many people do in a lifetime.
"We could not have more love and pride for our beautiful boy.
"His body, heart and soul may soon be gone, but his spirit will live on for eternity and he will make a difference to people's lives for years to come.
"As his mum and dad, we will make sure of that. We owe that to our boy.
"We will do our utmost to ensure that no parents have to go through what we have been through and the next Charlie that comes along will get this medicine before it's too late and Charlie will save many more lives in the future, no doubt about that.
"We are struggling to find any comfort or peace with all this, but one thing that does give us the slightest bit of comfort, is that we truly believe that Charlie may have been too special for this cruel world.
"We truly believe that any parent would have wanted the same thing if they knew what we knew.
"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."
She added: "Mummy and Daddy love you so much, Charlie, we always have and we always will and we are so sorry that we couldn't save you.
"We had the chance but we weren't allowed to give you that chance."
Ms Yates said the last 11 months had been the "best" and the "worst" of times.
"The last 11 nearly 12 months have been the best, the worst and ultimately life changing months of our lives but Charlie is Charlie and we wouldn't change him for the world," she said.
"All our efforts have been for him.
"This is one of the hardest things that we will ever have to say and we are about to do the hardest thing that we'll ever have to do which is to let our beautiful little Charlie go."
She added: "Put simply, this is about a sweet, gorgeous, innocent little boy who was born with a rare disease, who had a real, genuine chance at life and a family who love him so very dearly and that's why we fought so hard for him."
Ms Yates said a new scan had forced a change of heart.
"We are truly devastated to say that following the most recent MRI scan of Charlie's muscles ... as Charlie's devoted and loving parents 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."
Ms Yates said American and Italian medical teams were "still willing" to treat Charlie.
She said "they both agreed that treatment should have started sooner".
"He's not brain dead (and never has been).
"He still responds to us, even now, but after reviewing the recent muscle MRI it was considered that Charlie's muscles have deteriorated to the extent that it is largely irreversible and, were treatment to work, his quality of life would now not be one which we would want for our precious little boy," said Ms Yates.
"There is one simple reason for Charlie's muscles deteriorating to the extent they are in now - time. A whole lot of wasted time.
"Had Charlie been given the treatment sooner he would have had the potential to be a normal, healthy little boy.
"His muscles were in pretty good shape in January, although obviously weaker than a child of similar age, and his brain scan was that of a relatively normal child of his age.
"He may well have had some disabilities later on in life but his quality of life could have been improved greatly.
"The reason that treatment was not commenced in January or April this year was that Charlie was found to have 'irreversible brain damage' and treatment was considered as 'futile'."
But she said an American and an Italian specialist had reviewed scans done in January and April and had "confirmed" that they showed "no actual evidence of irreversible brain damage".
She said the American specialist, Michio Hirano, had not had access to the "raw data".
Ms Yates added: "Had we had the opportunity to have raw data of the (scans) independently reviewed, we are convinced Charlie would be on treatment now and improving all the time."
She went on: "However, we are now in July and 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."
Ms Yates said she hoped that Charlie's life "will not be in vain".
"We have always acted in our son's best interests," she said.
"We have always been led by Charlie.
"I promise every single one of you that we would not have fought this hard for our son if we thought that he was in pain or suffering.
"There has never been any proof that he was and we still don't think that he's in pain or suffering to this day.
"Having said that, we have decided to let our son go and that's for one reason and one reason only.
"It is because the prospect of improvement is unfortunately now too low for Charlie."
She added: "Due to the deterioration in his muscles, there is now no way back for Charlie. Time that has been wasted. It is time that has sadly gone against him."
Ms Yates added: "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. We have only been asking for a three-month trial of treatment to see if there was any improvement.
"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 what would have happened if he got treatment, but it's not about us. It's never been about us. It's about what's best for Charlie now. At the point in time when it has become too late for Charlie we have made the agonising decision to let him go.
"This has also never been about 'parents know best'. We have continuously listened to experts in this field and it has raised fundamental issues, ethically, legally and medically - this is why the story of one little boy from two normal everyday people has raised such conflicting opinions and ferocious arguments worldwide.
"All we wanted to do was take Charlie from one world-renowned hospital to another world-renowned hospital in the attempt to save his life and to be treated by the world leader in mitochondrial disease. We feel that we should have been trusted as parents to do so but we will always know in our hearts that we did the very best for Charlie and I hope that he is proud of us for fighting his corner.
"We will have to live with the 'what ifs' which will haunt us for the rest of our lives but we're thinking about what's best for our son. We have always believed that Charlie deserved a chance at life and we knew that his brain was not as bad it was made out to be and that's why we continued.
"We completely understand that everyone is entitled to their opinion and this was always going to be a matter which would cause a huge debate in who's right and who's wrong. In truth, there are no winners here.
"One thing is for sure though. We know deep within our hearts that we have always had Charlie's best interests in the forefront of our minds and despite what some people think of us, we will try to walk away from this with our heads held high.
"As I said, we know the truth, and in our hearts we know that we have done all of this for our darling little Charlie. We have never done this for selfish reasons. We didn't keep him alive just because we couldn't bear to lose him.
"Charlie had a real chance of getting better. It's now unfortunately too late for him but it's not too late for others with this horrible disease and other diseases.
"We will continue to help and support families of ill children and try and make Charlie live on in the lives of others.
"We owe it to him to not let his life be in vain."
Ms Yates thanked her lawyers, who had worked for free and "won't even let us buy them a coffee".
She also thanked everybody who has supported them "throughout this journey in this country and thousands of people worldwide".
Ms Yates added: "We also would like to thank the staff at Great Ormond Street Hospital who have looked after Charlie and kept him comfortable and stable for so long.
"The care he has received from the nurses who've cared for him has been second to none."
She went on: "Most of all, we would like to thank Charlie for the joy he has brought to our lives.
"The love we have for you is too much for words and we love you so very much."
Ms Yates ended her statement by saying: "Sweet dreams baby. Sleep tight our beautiful little boy. Charlie Matthew William Gard. Our hero!"
Ms Yates and Mr Gard hugged and kissed court staff, supporters and relatives as they left the court room.
Charlie's father later read part of the same statement on the steps of the Royal Courts of Justice, fighting backing tears as he paid tribute to the nurses and staff at Great Ormond Street who had "worked tirelessly" to care for their "warrior" son and telling the gathered press and media: "We will miss him terribly."