Britain's Queen Elizabeth II turns 90 today (21.04.16).

Britain's Queen Elizabeth II

Britain's Queen Elizabeth II

The monarch - who was born on April 21, 1926 - was crowned in 1953 and millions turned out for her coronation to see the young royal pledge her devotion to the United Kingdom and the commonwealth's 53 nations.

Tributes to the longest ruling monarch are trending on social media site Twitter with the hashtag "HappyBirthdayYourMajesty".

Her second son, Prince Andrew, Duke of York, aged 56, posted a photograph of his mother with her beloved pet Corgis and captioned the image on Twitter: "The Queen is 90! #HappyBirthdayYourMajesty"

Whilst the British Prime Minister, David Cameron, led messages of "happy birthday" and called the royal "a rock of strength".

He penned on the social media site: "The whole country will want to wish the Queen a happy birthday today - she has been a rock of strength for our nation"

However, Mr Cameron is expected to praise the Queen in a broadcast to air later today, saying she has "lived through extraordinary times".

He says: "Her Majesty The Queen has lived through some extraordinary times in our world.

"From the Second World War to the rations with which she bought the material for her wedding dress. From presenting the World Cup to England at Wembley in 1966 to man landing on the moon three years later. From the end of the Cold War to peace in Northern Ireland.

"Throughout it all, as the sands of culture shift and the tides of politics ebb and flow, Her Majesty has been steadfast - a rock of strength for our nation, for our Commonwealth and on many occasions for the whole world."

London Symphony Orchestra added to their "patron" on Twitter: "Wishing a very happy 90th birthday to our Patron Her Majesty The Queen. #HappyBirthdayYourMajesty

The Queen is expected to spend the day with her family in Windsor and her official Twitter page - with goes by the handle @BritishMonarchy - has revealed she is watching reels of old family footage with her son, the Prince of Wales.

They said: "The Queen and The Prince of Wales reminisce as they view private family footage together."

Elizabeth's 90th birthday also marks the release of three poignant portraits of her sitting with huge family, which includes her husband the Duke of Edinburgh, 94, the heir to the throne Charles, Prince of Wales, her daughter Princess Anne, 65, and her youngest son Prince Edward, Earl of Wessex, 52.

While the British media will celebrate with unique coverage today, other marks of celebration include a set of official stamps released by Royal Mail which will feature the image of the Queen and her two-year-old grandson Prince George, whose parents are the Duke and Duchess of Cambridge.

Meanwhile, celebrities have been sharing their memories of meeting Queen Elizabeth and British chef and TV personality Ainsley Harriott, 59, has recalled how he was often asked to cook for Princess Margaret and her sister the Queen in the 1980s, and would make traditional cod mash, cabbage and bacon for the royals.

Ainsley eventually met Queen Elizabeth in 1996 and told the BBC during a special broadcast titled 'The Day I Met The Queen': "Instantly, she made me feel comfortable. I remember her saying, 'I know what you do'. And in that instance, I wanted my mum to be there."

Speaking of his mother and father, Chester and Peppy Harriott - who hailed from the commonwealth country Jamaica - he continued: "Our late mother instilled a healthy respect for her majesty into myself and my sister and my mother was a royalist. I remember every Christmas time mum would make us pay attention to the Queen speech. There's a certain amount of respect that we have for what she's done for the country and our parents really recognised that."