David and Sam preparing for the patter of more tiny feet

She gave birth to their third child just eight months ago, but Samantha Cameron, the pretty wife of Tory leader David, is already planning another addition to their family.

The 34-year-old company director has told friends she is desperate to have another baby while her husband waits in the wings to become Prime Minister.

The couple's eldest child, Ivan, four, was born with severe epilepsy and cerebral palsy. The Camerons also have a three-year-old daughter, Nancy, and baby Arthur, who was born on Valentine's Day.

According to sources, Samantha, who works four days a week at Bond Street stationers Smythson, has said she wants to have a fourth child sooner rather than later.

"Sam is a very capable lady and the most important thing is her family," says a source. "She has always wanted a large family and a fourth child would be the icing on the cake.

"She and David dedicate a lot of time caring for Ivan, who requires regular hospital visits, but she still thinks she will have time to raise another child."

Despite her full-time job Samantha is said to be unfazed at the prospect of being a working mother of four.

It is thought that David, 40, who is constantly at pains to reinforce his vote-winning family-man image, is supportive of the decision and recently said in an interview: "The fact is whatever I do or don't achieve in politics, nothing matters as much as my family."

However, David has also admitted the births of Nancy and Arthur were overshadowed by Ivan's medical problems.

"Doctors are unable to say whether his condition is genetic. Dave and Sam know there will always be a risk when having a child but they are determined to forge ahead building their family," says my source.