Mr Blair accompanied her to London’s Chelsea and Westminster Hospital where she underwent an operation before being discharged last night.
The couple have delayed the start of their break in France, a Downing Street spokeswoman said.
Mrs Blair, who is 47, has four children, Euan, aged 17, 16-year-old Nicky, Kathryn, who is 14, and two-year-old Leo, who was the first baby to live in Downing Street in modern times after his surprise arrival in 2000.
A woman who is pregnant at the age of 47 has a 50% chance of miscarriage, the Royal College of Obstetricians and Gynaecologists said yesterday.
The prime minister’s wife was last seen in public at the closing ceremony of the Commonwealth Games, which took place in heavy rain in Manchester on Sunday night.
Downing Street released a statement in reply to inquiries from journalists awaiting the Blairs’ arrival in Toulouse. Their non-appearance had sparked speculation that Mr Blair could be delayed by matters concerning Iraq.
However, Number 10 said: “Following inquiries from the media in France and in London we can confirm that the prime minister and Mrs Blair have delayed their departure on holiday.
“This follows Mrs Blair’s admission to the Chelsea and Westminster Hospital last night for an operation after a miscarriage.
“She had an operation under general anaesthetic and was discharged this morning.
“The prime minister was at the hospital with her and they both would like to thank the hospital staff for their support and care.
“Mrs Blair came through the operation well and is now resting. The prime minister and Mrs Blair will join the rest of their family later for what they hope will be a quiet holiday.
“While we understand the inevitable interest in this matter we hope that the media will continue to allow the prime minister and his family to enjoy as normal a family holiday as possible.”
Number 10 declined to say where the Blairs were staying before heading out to France.
Mrs Blair is understood to have known she was pregnant.