Prime Minister Tony Blair has paid a glowing 80th birthday tribute to the Queen by describing her as "one of the most respected people of our times".
The Queen will be 80 on Friday
Her sense of duty and service "had a profound impact on our country, the Commonwealth and the world", he said.
He praised her as a "remarkable source of constancy and strength", saying she had met the changes in the world "with extraordinary grace and dedication".
"She has been a reassuring and unifying presence for her people," he said.
Mr Blair, the 10th prime minister to serve during her reign, said she was a "unique" figure.
"Like each of my predecessors I am profoundly grateful for her wise counsel," he said.
"She has superb judgement, an intuitive empathy with people and above all an unshakeable and profound sense of duty.
"It is this sense of duty which motivates her, defines her reign and since it communicates itself unobtrusively but none the less obviously to her subjects, brings her I believe the love of the people of this country."
During her 80 years the country had witnessed the horrors of the Second World War and "celebrated extraordinary triumphs", said Mr Blair.
"Throughout, as part of the Royal Family and as the Queen, she has been a reassuring and unifying presence for her people," he said.
Mr Blair said the Queen showed no sign of slowing down, having recently completed her 14th tour of Australia, including officially opening the Commonwealth Games.
In her 50 plus years on the throne, she had undertaken 256 official overseas visits to 129 different countries, he said.
"I know I speak for the whole House and a grateful nation when I can say to Her Majesty - long may she reign over us."
Conservative leader David Cameron joined in the tributes, describing the landmark of the Queen's 80th birthday "as a genuine cause for national celebration".
"She has been our Queen for 54 years. Through a thousand years of British history, only Queen Victoria, George III, Henry III and - by a few months - Edward III have sat on the throne for a greater period," he said.
However, that comment prompted Labour and Lib Dem MPs to question Mr Cameron's recollection of his history lessons - as the Queen has already overtaken the 50 years of Edward III's reign between 1327 and 1377.
Liberal Democrat Bob Russell (Colchester) said outside the Commons: "I am sure the history department at Eton must be mortified that one of their more famous students is clearly in need of extra-curricular tuition in history."
Labour's Louise Ellman (Liverpool Riverside) said: "What must his history master be thinking?"
'God save the Queen'
In his Commons tribute, Liberal Democrat leader Sir Menzies Campbell said the Queen was not only the UK's head of state, but "the embodiment of the nation".
He said her Silver and Golden Jubilees "were marked by the warmth and affection of the people of Great Britain".
"Her Majesty has truly shown herself to be a steadfast guardian of the people, of the Commonwealth, and of our democratic traditions.
"And when we celebrate her birthday, we celebrate the values which illuminate her life. God save the Queen."