Students may still be carrying Covid even if they test negative and should exercise caution when visiting friends and family over Christmas, experts have said.

Universities, using lateral flow devices which give a quick result, have begun mass testing students ahead of their return home.

But figures released by the Government this week showed that mass testing with similar devices in Liverpool had missed positive cases 50 per cent of the time, meaning some students will think they are virus-free when they are actually infected. 

Dr Daniel Howdon, a researcher in health economics at the University of Leeds, said it was "questionable" whether mass testing should continue.

"The stuff put out by higher education institutions on this has tended towards indicating that students are safe if they test negative or test negative twice," he said. "So there is clearly a potential for false reassurance provided by a false negative test result.

"I think a large number of students undertaking this test will assume that they are safe if they test negative twice, and assume they can visit elderly and vulnerable friends and relatives. We just do not know how safe they are, nor how they will react."

Universities are opening temporary testing centres at which hundreds of thousands of students will be checked for Covid before they leave for the holidays. Students have been asked to take two tests, three days apart.

But results published by the Government this week from the pilot scheme in Liverpool (see video below) show that, compared to the usual polymerase chain reaction (PCR) tests, lateral flow tests picked up just five out of 10 positive cases.

UCU, the lecturers’ union, said the tests were "nowhere near accurate enough" to allow students to travel to see vulnerable family members this Christmas.

Jo Grady, its general secretary, said: "The results from Liverpool add to the mounting body of evidence showing we were right to raise our concerns with the use of lateral flow tests on students last month.

"These tests are nowhere near accurate enough to allow hundreds of thousands of students to move across the country, mix with multiple households and then return to university for in-person teaching next term. The Government needs to be honest about how unreliable these tests are so people do not put their faith in them in order to spend time with vulnerable loved ones over Christmas."

Universities UK also urged students to take precautions when interacting with other people even if they test negative, and a spoeksman said: "As we have heard in recent days, we all need to think carefully about our actions and interactions over the Christmas period, and that goes for everyone whether you have had a test or not."

A Department of Health and Social Care spokesman said: "The country’s leading scientists have rigorously evaluated the Optigene LAMP test and Lateral Flow Test and confirmed the accuracy of the tests for asymptomatic testing."