Prince Philip left hospital on Sunday after undergoing treatment for a chest infection.
Television channels showed the Duke of Edinburgh, who had been in the King Edward VII hospital in central London since Thursday, walking to a car to be driven to Windsor Castle.
His diary engagements for Monday have been cancelled and future dates will be decided on a day-to-day basis.
The blunt-talking prince, who married Elizabeth in 1947 and gave up his naval career four years later, has been a pivotal figure in the House of Windsor for more than five decades.
He has attracted unwelcome headlines over the past six months at the inquest into the death of his former daughter-in-law Princess Diana, who died in a 1997 Paris car crash with her lover Dodi al-Fayed.
Dodi's father, luxury storeowner Mohamed al-Fayed, alleges that his son and Diana were killed by British security services on the orders of Prince Philip.
Lord Justice Scott Baker, presiding judge at the inquest, said there was "not a shred of evidence" to back al-Fayed's conspiracy theory.
Over the years, Prince Philip has won notoriety for a string of gaffes on world tours. He told British students in China: "If you stay here much longer, you'll be slitty-eyed."
