Margaret Thatcher has praised the Queen as an "inspiration" and an example to the nation in an 80th birthday tribute.
The Queen was a guest at Baroness Thatcher's 80th birthday party
In a rare broadcast, the former prime minister said she had been privileged to take advice from the Queen during her 11 years in Downing Street.
"She brings to bear a formidable grasp of current issues and a tremendous breadth of experience," Baroness Thatcher told ITV News.
Wishing the Queen happy birthday, she declared: "Long may she rule."
As prime minister, Lady Thatcher had weekly audiences with the Queen.
There had been reports that the two women did not get on well when Lady Thatcher was in power.
But the Queen was a guest at the former prime minister's 80th birthday party last year.
On Thursday, Lady Thatcher said: "I am delighted to be able to send this message, on the occasion of Her Majesty's 80th birthday.
"This is a great day - a day when we can express our boundless admiration and affection for our Queen.
"She is truly an inspiration and an example to the whole nation. Long may she rule. Happy Birthday Ma'am."
Lady Thatcher said the Queen was "eminently accomplished" at putting people at ease, something she did at their first audience in 1979.
"But anyone who believes that such meetings are a mere formality would be greatly mistaken," she said.
"The Queen takes an intense interest in every aspect of life in our country."