Schools in Merton will use sex education lessons to warn pupils that delaying parenthood could make it harder for them to have children.

A dozen schools will be introducing a fertility module into sex and relationship education classes for 16 to 18-year-olds.

Teachers will tell girls that they should start a family in their late twenties or early thirties to avoid disappointment and boys that smoking and STIs can affect their fertility.

Geeta Nargund, from the charity Create Health Foundation who are funding the initiative, said: "There has been a lot of enthusiasm (for the lessons) including from a head teacher from a boys' only school.

"We are keen to get started and later roll out the initiative to mover schools and eventually ask the Department of Education to fund it."

Dr Nargund, a senior consultant in reproductive medicine at St George's Hospital in Tooting, says she has seen a growing number of "career-women" putting off having children to their thirties and forties only to be "shocked" when they fail to conceive naturally.

But critics have blasted the move, saying it will put more pressure on young women.

Clare Murphy, from the British Pregnancy Advisory, said: "The risks of waiting longer to start a family should never be overstated, and where older motherhood may carry slightly increased medical risks, the answer is not to put young women under more pressure about their biological clocks."

Stephen Alambritis, the leader of Merton Council, said: "We owe it to our older pupils to give them as much information as possible."