'I got pregnant with Leo because I was too embarrassed to take contraception to Balmoral' - Cherie

She says she became pregnant with her fourth child in September 1999, during the annual Prime Ministerial visit to Balmoral.

The previous year, Mrs Blair writes in her autobiography, she had been horrified to find all her belongings unpacked, including the contents of her toilet bag.

In 1999, out of "sheer embarrassment", she left her contraception behind.

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That night, she writes: "As usual up there, it had been bitterly cold, and what with one thing and another ..."

When her period failed to arrive, she writes: "I thought, I can't be. I'm too old. It must be the menopause."

Her fitness guru Carole Caplin bought a pregnancy testing kit and when it was positive Mr Blair's first reaction was "Oh my God", followed by "We'll have to tell Alastair [Campbell, his communications chief].'

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She says Mr Blair's reaction to the birth of their son Leo in May the following year was also to say: "We need to ring Alastair".

While Mrs Blair told her close family, her husband told Anji Hunter, his long-time aide and Gordon Brown, because he was sensitive to the Chancellor's feelings.

Mr Blair later told her: "You have to understand, Cherie. It's a very sensitive topic for him. The whole issue of my being a family man is very sensitive to him."