US president Barack Obama has praised the Queen for her "generous human touch" as he credited the monarch with helping to ensure the lasting special relationship between the UK and America.
The US leader described the sovereign as a "source of strength and inspiration" for people in Britain and beyond.
Mr Obama features in ITV's Our Queen At Ninety documentary, in which viewers will also hear from the head of state herself.
The US president said: "I'm glad to be the very first President of the United States to wish Her Majesty a happy 90th birthday.
"For nine decades Her Majesty has been a source of strength and inspiration, not only for the people of Britain, but for millions of people around the world.
"And when the Queen hosted Michelle and me at Buckingham Palace we saw the same grace and good humour, that generous human touch, that the people of the United Kingdom have loved for generations."
He added: "We often speak of the deep and enduring partnership between our two countries. It is indeed a special relationship.
"And Her Majesty has been a vital part of what keeps our relationship so special.
"The United States is extremely grateful for her steady and energetic leadership and we look forward to Her Majesty's continued service in the years to come. Happy birthday, your Majesty."
The documentary will feature a number of people paying tribute to the Queen, including her daughter-in-law the Duchess of Cornwall, who described the fact the sovereign is still a keen horse rider as she enters her 91st year as "incredible".
She said: "Horses respond to her, they love her. It's a two-way relationship, they both love each other."
In the programme, the Queen is heard to tell a member of staff: "The weather forecast is appalling ... It's quite difficult at this time of year ... and I'm rather a fairweather rider at the moment, I don't like getting cold and wet."
:: ITV's Our Queen At Ninety airs on Easter Sunday at 8pm.