Irish Royal Guards toast royal birth with a glass of stout
Army units connected to Prince William toasted the birth with a traditional glass of Irish stout at their barracks in Aldershot and at bases in London.
The Ministry of Defence said other units connected to WIlliam also marked the birth, including the Army regiment he was commissioned into from Sandhurst, the Household Cavalry, which is partly deployed in Afghanistan.
Gun salutes will sound across London to mark the birth of the royal baby as the armed forces join in the celebrations.
Gun salutes are fired for the birth of every prince or princess, no matter where their place is in the line of succession, the British Ministry of Defence said. The last royal salute for a birth was for Princess Eugenie in 1990.
The King's Troop Royal Horse Artillery, wearing full dress uniform, will ride past Buckingham Palace to Green Park where they will stage a 41-gun royal salute.
71 horses will pull six First World War-era 13-pounder field guns into position for the royal salute at 2pm.
Each of the six guns will fire blank artillery rounds at 10-second intervals until 41 shots have been fired.
Whilst a royal gun salute normally comprises 21 guns, this is increased to 41 if fired from a royal park or residence. Uniquely, at the Tower of London, which is a royal residence, 62 rounds are fired as this also includes an additional 21 guns for the citizens of the City of London to show their loyalty to the monarch.
Celebrations are not confined to London, as all of the Queen's ships - in home or international waters - are flying the Royal Navy Ensign flag from their mast to mark the birth of the prince. HMS Lancaster, deployed on a patrol task in the Caribbean, added its own congratulatory message to the Duke and Duchess, spelling out "It's A Boy!" on the deck of the ship. Commanding officer Commander Steve Moorhouse said: "As 'the Queen's Frigate' we were particularly delighted to hear the fantastic news of the royal birth and everybody on board HMS Lancaster sends their warmest congratulations and best wishes to the Duke and Duchess of Cambridge."
Staff from RAF Valley, where Prince William serves as a search and rescue pilot, were among the first to offer their congratulations. Station commander Group Captain Adrian Hill sent a congratulatory message to the couple on behalf of RAF Valley. "We're all delighted to learn that Flight Lieutenant Wales and the Duchess of Cambridge are now proud parents of a newborn son," he said. "We wish them all the very best for the future. From everyone at RAF Valley, congratulations."