Prince Harry is reportedly isolating at home with Meghan and Archie in case Prince Philip's condition worsens.
Sources say the Duke is staying at home in case he needs to board a flight home at short notice should the worst happen, with a private jet ready to take him back to the UK, according to reports from the Mirror UK.
Harry is being kept informed of his grandfather's condition and will be tested for Covid-19 before leaving the US and entering the UK, it's understood.
Members of the royal family have special dispensation to travel in this case, meaning Harry would not have to quarantine in the UK if he tested negative.
Meanwhile, Meghan would face the decision whether or not to travel, as it's understood she is about five months pregnant.
A source said, "Harry is of course concerned for his grandfather's welfare and is being kept regularly informed on his condition.
"He has chosen to stay in case he is required to return to the UK at short notice."
Prince Philip is preparing for a third night in hospital away from the Queen after he was admitted "as a precaution" on Tuesday.
The Duke of Edinburgh will stay at King Edward VII's Hospital in London tonight for undisclosed health reasons, unrelated to coronavirus, the Daily Mail reports.
Sources say the "sprightly" 99-year-old was "in very good humour" before being admitted after a "short period" of feeling ill.
It's the third night he's spent away from the Queen after it was revealed they "enjoyed lunch and dinner together" in Windsor most days as the coronavirus lockdown gave them the perfect opportunity to spend more time together.
And a royal biographer has claimed the "blunt" Prince probably isn't the perfect patient as he "doesn't like people making a fuss over him".
Penny Junor said jokingly that medical staff at the private London hospital probably "would not want" the Duke on their shift as he "gets irritated" when people make a fuss over him.
He was admitted to hospital earlier this week as a precaution after feeling unwell.
Buckingham Palace said on Wednesday that the Queen's husband was admitted to the private King Edward VII Hospital on Tuesday evening. He was expected to stay in hospital for a few days of "observation and rest".
His illness is said to be unrelated to Covid-19. He and the Queen both received their first dose of a coronavirus vaccine earlier last month.
Philip retired from public duties in 2017 and rarely appears out and about, with his most recent engagement a military ceremony at Buckingham Palace last July.
The Duke has been staying at Windsor Castle with the Queen amid the coronavirus lockdown. Plans are currently underway for his 100th birthday celebrations on June 10.
Philip married Elizabeth in 1947 before she became Queen and is the longest-serving royal consort in British history. Together they have four children, eight grandchildren and nine great-grandchildren.