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.
"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."