It is often described as one of life's greatest joys - the profound experience of becoming a parent.
But for several men across the UK, it has become irreversibly tainted with inexplicable grief, anger and resentment.
A growing number of fathers are being subjected to the betrayal of learning that their 'child' is, in actual fact, not biologically their own.
They had been present for the birth and held back tears as they cradled their beloved son or daughter in their arms for the first time. Over a number of years, they emotionally and financially invested in their future.
But then, suspicions slowly start to mount about their relation to the child.
