Couples undergoing IVF in the UK are exploiting an apparent legal loophole to rank their embryos based on genetic predictions of IQ, height and health, the Guardian has learned.

The controversial screening technique, which scores embryos based on their DNA, is not permitted at UK fertility clinics and critics have raised scientific and ethical objections, saying the method is unproven. But under data protection laws, patients can – and in some cases have – demanded their embryos’ raw genetic data and sent it abroad for analysis in an effort to have smarter, healthier children.

Dr Cristina Hickman, a senior embryologist and founder of Avenues fertility clinic in London, said rapid advances in embryo screening techniques and the recent launch of several US companies offering so-called polygenic screening had left clinics facing “legal and ethical confusion”.

“This opens a whole can of worms,” said Hickman, who raised the issue in a letter last month to the Human Fertilisation and Embryology Authority (HFEA).

One US company, Herasight, which charges couples $50,000 (£37,000) to assess an unlimited number of embryos, confirmed that it had already worked with couples undergoing IVF at clinics in the UK. There is no suggestion Herasight is in breach of any regulations.

Avenues currently has two patients who – without the clinic’s involvement – intend to use Herasight’s service. One, a 29-year-old woman, said she and her husband hoped to reduce the risk of diseases, such as diabetes, and to pick embryos with high predicted IQ.

“People are willing to spend loads of money and heartache to give their kids slightly better lives after they’re born,” said the patient, who wished to remain anonymous. “This seems the best bang for your buck; it’s less per year than private school.”

Herasight claims to deliver an average gain of six IQ points for a couple with five embryos and provides sex, predicted height and risk scores for conditions including heart disease, common cancers, Alzheimer’s and schizophrenia. The patient, who has banked 16 embryos, said: “I’m hoping to have six where I’m like: ‘Wow, that’s an excellent profile.’”

In the UK, tests performed on embryos are legally restricted to a list of serious health conditions, such as Huntington’s, sickle cell disease or cystic fibrosis. Clinics cannot perform polygenic screening for the purpose of embryo selection. The HFEA also says a clinician should not be guided by a patient on which embryo to use based on that patient’s knowledge of its scores.

Peter Thompson, the chief executive of the HFEA, said polygenic testing was unlawful for use in the UK. “Licensed clinics in the UK are responsible for selecting embryos based on what is permitted in the HFE Act and therefore should not offer such testing and subsequent treatment,” he said.

“However, there is nothing to stop a UK-based couple seeking such testing, and indeed treatment, overseas, but a UK licensed clinic should not then make decisions on what embryo to put back using that information.”

Some have questioned whether the HFEA’s position is enforceable. Hickman said the clinic would not generally block a couple’s request that a particular embryo is transferred, provided there was no conflict with medical safety.

“If a patient wants to have embryo number one transferred and the clinic says no, we want embryo number three, I cannot think that, if this were to go to a judge, they would say no, you have to transfer embryo number three,” she said.

“I would rather have [polygenic testing] allowed here and have the HFEA control how to do it ethically.”

Herasight has built its algorithms using large genetic databases, including UK Biobank, and has published the scientific basis for its predictions. However, using polygenic scores in the context of embryo selection remains contentious and the European Society of Genetics has condemned the technique as “unproven and unethical”.

There are also broader ethical concerns, including the potential for a stratified society where wealthier people pay to select the embryos they prefer and the prospect of normalising a belief that some individuals are genetically superior.

Prof Angus Clarke, a clinical geneticist at Cardiff University, said: “These companies are dealing in murky science in an emotionally fraught context.” He said some parents may find it hard to resist the promise of selecting the “best child possible rather than any old child”.

“Let’s hope they’re not disappointed,” he added. “It might be difficult to live up to those expectations, if you’re that child.”

In a statement, Herasight said: “We can confirm that Herasight has been working with parents from across the globe, including families who have undergone IVF treatment in the UK, by transforming routine PGT-A data that parents have legally obtained and provided to us. This data can be accessed by clients under applicable data protection laws in the UK, Europe and the rest of the world.”

Herasight said it did not work with, or provide data to, IVF clinics or influence them in the selection of embryos.