Gun salutes as Queen Elizabeth II turns 89

London - Gun salutes rang out in London on Tuesday to mark Queen Elizabeth II's 89th birthday, as Britain's royals eagerly await the newest addition to the family.

A 41-gun salute was fired at midday in Hyde Park, with a 62-gun salute following from the Tower of London - and it could happen all over again within days to celebrate the birth of Prince William and his wife Kate's second baby.

The queen's fifth great-grandchild, who will be born fourth in line to the throne, is due later this month.

The monarch, who has been on the throne since 1952, was spending the day at Windsor Castle, west of London.

Several hundred passers-by watched as horses from The King's Troop Royal Horse Artillery galloped across Hyde Park, pulling six rare World War I-era 13-pounder field guns.

With the guns' metal gleaming in the sun, the soldiers fired blank rounds at 10-second intervals, the boom echoing off towers around the city as white smoke billowed.

The ceremonial display took place next to Park Lane, home to some of London's plushest hotels, and less than a kilometre from 17 Bruton Street, where the queen was born in 1926.

The London townhouse of her mother's parents has since been demolished and a plush Cantonese restaurant now holds that address.

The standard royal salute is 21 rounds, with an extra 20 when fired from a royal park or residence.

At the Tower of London, a further 21 were fired over the River Thames to show the city's loyalty to the sovereign.

Traditionally, the monarch has an "official" birthday in the summer months because the weather is supposed to be better for open-air celebrations.

Queen Elizabeth II has been on the throne for more than 63 years and on Sept 9, she will overtake Queen Victoria to become the longest reigning monarch in British history.