One person in 10 is mistaken about the identity of their father, genetic tests for hereditary illnesses are revealing, according to an NHS chief.

The era of genomic medicine is allowing doctors to screen rising numbers for preventative action against diseases such as cancer and Alzheimer’s.

About 220,000 such tests are carried out by the NHS in England and Scotland each year.

But Ian Cumming, head of Health Education England, the NHS training body, said hospitals were being left in an ethical quandary as they were uncovering some awkward family secrets.

It is currently estimated that around 4 per cent of the population are unaware the man they call their father is not their true biological relative.

Mr Cumming told the Hay Festival that within a decade everyone who wanted to be genome tested could be.

“But it is not without controversy,” he warned. “If you look at people who have had genetic tests within families for reasons other than trying to work out paternity, for one in 10 people your dad isn’t who you think it is.”

He said this was the dilemma: “Are we going to tell people: ‘That’s not your dad’ – or are we going to keep that information to ourselves? I don’t think that would be acceptable ethically.”