The chemist Boots caused unease in some quarters last week when it became the first high-street chain in the UK to sell over-the-counter paternity kits.

The decision responds to demand by consumers, although many medical experts are worried that people will obtain genetic test results that could impact hugely on them and their families without any proper support.

The kit's manufacturer, in turn, argues that a majority of the tests will be carried out on very young children before the results could have any impact on their understanding.

The result is not instant, as with a home pregnancy kit, but has to be sent off to a lab. Nor, at £29.99 for the kit and £129 for the processing, is it cheap. The justification, say the manufacturers, is that an estimated one in 25 men, according to research in the UK and US, is not the biological parent of a child he believes he fathered. In the US, 31% of men who undergo paternity testing find out they are not the father.

In Britain about 50,000 children born every year are registered without a father being named on the birth certificate, something the government has been trying to tackle. However, paternity testing is not available on the NHS, even if ordered by a court.

Josephine Quintavalle, director of Comment on Reproductive Ethics, was critical of the paternity kit. "It will be used almost exclusively as weaponry between battling adults in fraught relationships, to the detriment of the unfortunate child caught in the middle," she told Register, an online science journal. "This child will be unable to give informed consent to the use of his or her tissue, but the emotional impact of the test results, which could include paternal rejection, are likely to be devastating."

But others, including the Fatherhood Institute, welcomed the test: "If science can help to clarify parentage, then it would be perverse to argue that this is not a good thing as long as these tests are done with proper safeguards for the privacy of the parents," it said.

The kits at Boots do require the presumed father, mother and children over 16 to sign consent forms (with the mother signing for young children), as well as proof of identification – all measures that can perhaps be circumvented by a determined individual, although since 2006 it has been illegal to take someone's DNA without permission.

Boots says the kit meets a demand for an easy to use, reliable test made and analysed in the UK. Previously, kits had to be sent to the US for analyis.

Once the buyer has collected all three mouth swabs and sealed them into special bags, they are posted to Anglia DNA Services laboratories for analysis. The manufacturers say the test is 99.9% accurate for a positive match, and 100% accurate for a negative result.

Dr Mandy Hartley, Anglia DNA's technical manager, said the tests provided "peace of mind" for families trying to move on with their lives. The firm processes 3,000 tests a year, but this is expected to rise significantly as Boots offers the kits at 375 stores.

A spokeswoman at Anglia DNA said that for most people there was rarely a surprise in the results: "Every paternity issue is different, but for the majority of cases families receive the results they were expecting."