By Richard Greene and Hilary Clarke

CNN

(CNN) -- The parents of the terminally ill British baby Charlie Gard have given up their legal fight over treatment for their son in the US.

Their lawyer Grant Armstrong told the UK High Court Monday that experts have said that the "window of opportunity no longer exists."

"For Charlie, it is too late...treatment cannot offer a chance of success," he told the court.

Charlie's parents Chris Gard and Connie Yates made their decision following the latest medical reports and scans, the Press Association reported.

"Dark days lie ahead for these parents..." they want to spend time with Charlie," Armstrong said.

Rare genetic mutation

Charlie, whose first birthday is August 4, was born with a genetic mutation called mitochondrial DNA depletion syndrome. It leads to weakened muscles and organ dysfunction, among other symptoms. The prognosis is poor for most patients. Charlie is unable to breathe or move on his own.

Great Ormond Street Hospital -- where Charlie has been since October 2016 -- received permission from the European Court of Human Rights in June to discontinue life support.

But his parents wanted to bring him to the US for an experimental therapy called nucleoside bypass therapy. It has never been used to treat Charlie's form of mitochondrial DNA depletion syndrome.

The doctors at GOSH have argued that every medical option has already been considered and treatment in another country would not be in Charlie's best interest.

"These parents should know that no parent could have done more than they did for their child," said Justice Nicholas Francis, who is presiding over the case.

He was critical of those who weighed in on the case without knowing the facts. Both the Pope and US President Donald Trump have shown their support for Charlie's parents.

"When cases such as this go viral, the watching world feels entitled to weigh in regardless of whether or not they are informed on the facts of the case," he said.