Government lawyers have said any legal challenge against making low-paid people take confirmatory PCR tests to get financial support during isolation would stand a high chance of success, the Guardian has learned. Legal advice circulated within the UK Health and Security Agency (UKHSA) is understood to warn that there is a risk of a challenge under equalities law. The advice puts the chance of any challenge being successful at 50% to 70%. It says it could be argued that lower socioeconomic groups would be disadvantaged by the move to scrap confirmatory PCR Covid tests for everyone, if low-paid people were still required to get them in order to access £500 payments for support during isolation. “There is at least a medium-high 50-70% risk of successful challenge in the basis of equality and rationality grounds if we require individuals to have a confirmatory PCR to access TTSP [test-and-trace support] payments,” the advice says. “This is because it disproportionately impacts individuals from low-income backgrounds and if [lateral flow tests] are deemed to be sufficient from a public health and fraud perspective then to require all individuals to legally self-isolate then it is difficult to argue they should then [need a PCR test to] get access to the TTSP.” The move to scrap confirmatory PCR tests for asymptomatic people who test positive by self-reported lateral flow was announced this week. The exception is people on low pay who want to apply for Covid support payments, who are required to get confirmation because of the risk of fraudulent applications. It was also announced this week that PCR requirements would be scrapped for travellers arriving in the UK – a move that was opposed by the health secretary, Sajid Javid, on the grounds that it could set back detection of new variants coming in to the country. The Guardian revealed this week that Jenny Harries, the head of the UKHSA, had raised concerns about low-paid workers being disadvantaged by changes to the Covid testing regime in England. In a memo before Christmas, she highlighted a greater risk of false negatives for those on lower incomes forced to go to a testing centre to ensure they received the £500 Covid test-and-trace support payments. Harries wrote: “In higher prevalence periods, the abandonment of confirmatory PCR would mean that the people most likely to receive a false negative and therefore potentially become ill and then seek hospital care later and therefore more likely to die would be more likely to be those from lower socioeconomic groups who also have higher risk of underlying health conditions as additional risk factors.” The government made the changes partly to ease pressure on the PCR testing system while Covid case numbers are at an unprecedented level, and partly to allow people to start the clock on isolation periods earlier. However, there were also concerns among public health officials that a confirmatory PCR was counterproductive because of the risk of false negatives. Some experts have previously highlighted that there is still a reasonable chance of a person having Covid if they have a positive lateral flow followed by a negative PCR, with the PCR missing a positive result about one in 20 times. It is understood that data prepared for Javid this month contained estimates that about 9,000 out of 195,000 confirmatory PCR tests taken in November in England were false negatives. Out of 20,000 negatives overall, this suggested that 45% of all negative PCR results after a positive lateral flow result were false. In Wales, support payments are given out for self-reported tests without the need for a confirmatory PCR. It is difficult to quantify the number of people affected, but the House of Commons library found that about 370,000 support payments had been made by November. That means about 1 million applications are likely to have been made, because, based on previous data, the success rate is estimated to be about a third. The number of payments is likely to have risen again substantially during the latest wave of Covid. The UKHSA declined to comment on the leaked advice.