Tory leader's wife set to hit election trail with a bump

The news comes as the 38-year-old prepares to play a major role for the Tories in the general election campaign.

The couple, who are said to be "completely thrilled", already have two children – four-year-old Arthur Elwen and Nancy, six – but their severely disabled son Ivan died last year aged six.

Mr Cameron, 43, has given the impression of being broody in recent times, admitting on a visit to Calderdale Royal Hospital in Halifax in December that meeting a newborn baby "makes me want another one".

Prime Minister Gordon Brown and Lib Dem leader Nick Clegg both sent their congratulations.