A woman was given the wrong donor sperm at an Essex clinic due to a "mix up with paperwork".

The mix-up happened at Simply Fertility in Chelmsford and led to a couple conceiving a child via a sperm donor they had not wanted, according to a report by the Human Fertilisation and Embryology Authority (HFEA).

The couple originally underwent fertility treatment in 2016 at the clinic using sperm from one donor but this was unsuccessful.

A second cycle using a different sperm donor worked and resulted in the birth of a child.

The couple then went back to the clinic in 2018 and asked to use the sperm again from the donor in the second cycle in order to give their child a full genetic sibling.

But because of a mix up with paperwork, sperm from the first unsuccessful treatment was used. The mistake was only realised after the embryo had been transferred to the woman.

The HFEA report said the senior management team at the centre was informed immediately, and the couple were invited to the centre where the error was explained, and an apology provided alongside the offer of further support.

The couple reported a positive pregnancy test and said they intended to continue with the pregnancy.

The clinic has since taken action to ensure such a mistake does not happen again.

A statement from the clinic said: "Simply Fertility has maintained an exceptionally high standard of service since it opened in 2013.

"Unfortunately, adverse incidents do sometimes occur. As a part of our investigation into this matter, we have identified how the mistake arose and we have put in place measures to ensure this does not happen again.

"We cannot discuss the details of the matter due to patient confidentiality but we have of course apologised sincerely for the distress caused to those concerned."

The incident is detailed in a separate study published by the HFEA which showed that mistakes are rare - affecting less than 1 per cent of fertility treatment cycles - but have risen 6 per cent over the last year and 18 per cent in three years.

An increasing proportion of mistakes are serious, causing severe or moderate harm to patients.

Some of the increase is down to fertility clinics improving reporting of their errors, the HFEA said.

Sally Cheshire, chairwoman of the HFEA, said: "I'm pleased that this report indicates continued good performance across the UK fertility sector.

"Significant improvements in some of the key areas we've highlighted previously with clinics are especially reassuring, proving that working together with clinics and the professional bodies is having a positive impact for patients.

"It's good news that clinics have reduced the number of minor incidents, but we're concerned that any incident is one too many.

"We will continue to ensure that the whole sector learns from any clinic incident, however minor, to understand what went wrong and, crucially, that steps are taken to ensure it does not happen again."