A mother has warned of the risk of children spending hundreds of pounds on "free" online games available through Facebook after her 12-year-old son ran up bills of more than £900 without her knowledge.

The woman, who prefers to remain anonymous, discovered last month that her son had spent more than £900 on FarmVille. He had emptied his own savings account of £288 and had used her credit card to the tune of £625 to pay the bills.

FarmVille, one of the most popular games on Facebook, allows members to manage a virtual farm by planting, growing and harvesting crops, trees and livestock. New users are given virtual coins to set up their farm, and the revenue from matured crops can be used to maintain it. But those who are desperate to progress more quickly can buy extra virtual coins using real cash.