A couple undergoing fertility treatment had a baby after being given sperm from the wrong donor.
They decided to go ahead with the pregnancy after being told of the error.
It was revealed in a report by the Human Fertilisation and Embryology Authority (HFEA) which shows mistakes are on the rise.
The couple first used a sperm donor in 2016 but the treatment was unsuccessful.
A second cycle using a different donor led to them having a child.
They returned to the clinic in 2018 asking to use the second donor again in the hope of giving their a full genetic sibling.
But, because of a mix-up with paperwork at the centre in Chelmsford, sperm from the first donor was used.
The mistake was only realised after the embryo had been successfully transferred to the woman.
The HFEA report says mistakes affect less than 1% of fertility treatment cycles, but the number of errors has risen 18% in three years.
And the number of incidents classed in the most serious two categories has risen while minor issues have decreased.
In 2018/19, there were 606 mistakes – up from 571 the year before and from 514 in 2015/16 – of which two were grade A and 294 were grade B.
Grade A incidents include severe harm, such as being implanted with the wrong embryo.
Grade B errors include serious harm an individual, such as the loss or damage of embryos.
Sally Cheshire, who chairs the HFEA, said any mistake was “one too many”.
She said: “We will continue to ensure the whole sector learns from any clinic incident, [so] that steps are taken to ensure it does not happen again.”
