Record delays in Scotland's contact tracing system could be causing a "bigger spread of the virus", the country's national clinical director said as he admitted it was "straining" to cope with a surge in Covid cases.

       Prof Jason Leitch denied that Test and Protect was "collapsing" after official figures showed that fewer than a third of people who tested positive were interviewed about their close contacts within 24 hours.

       But speaking after the number of daily cases rocketed to almost 4,000, Prof Leitch said it was "a lot" for the system to cope with, adding that the delays before carriers and their contacts were interviewed "stretch when the numbers get big".

       The national clinical director also admitted that the delays in getting in touch with infected people, interviewing them and finding their close contacts could be helping the virus spread further.

       Labour said his comments confirmed that SNP "mismanagement" of the system was helping the virus spread and argued that the spread of infections was now out of control.

       Public Health Scotland figures showed that only 29 per cent of people who tested positive had spoken to a contact tracer within 24 hours of being logged in the Test and Protect system. This was by far the lowest proportion on record and down from 50 per cent recorded the previous week. Even at the peak of the second wave in January, it did not fall below 75 per cent.

       The proportion of virus carriers waiting more than 48 hours to be contacted surged from 15 per cent in the week ending June 20 to 28 per cent the following week.

       Tracing was also taking longer to complete, with only a fifth of cases being deemed "closed" within 24 hours because all close contacts had been interviewed. This was again a record low, and a sharp drop on the 80 per cent of cases concluded within a day in April.

       In an effort to tackle the backlog of cases and alleviate staff workloads, Test and Protect has started texting travellers arriving from red list countries, who are required to quarantine, rather than calling them. Teams are also asking those who have tested positive fewer questions about their travel and events.

       However, opposition politicians have demanded that the SNP provide the system with more resources.

       Jackie Baillie, Scottish Labour's health spokesman, said: "Jason Leitch's comments confirmed what we have known all along – that the SNP's mismanagement of Test and Protect is helping the virus to spread.

       "Test and Protect is collapsing under the pressure as cases spiral, and things will only get worse if we don't fix this. Ministers are cutting corners all over the place to try and keep the system afloat, but that will only make things worse."

       Prof Leitch told BBC Radio Scotland: "It's not collapsing, not by any means. In fact there are thousands of my colleagues around the country today doing contact tracing and talking to positive cases and doing a fantastic job. But you can't have 4,000 cases without it straining. That's a lot."

       Asked whether the delays were helping Covid spread, he said: "Potentially it does lead to a bigger spread of the virus. We would ask people to be really, really cautious – if you know you've been in touch with a positive case, then use your common sense and be careful.

       "But contact tracing is still getting in touch with people inside the WHO [World Health Organisation] recommended time. But you're right, those times of course stretch when the numbers get big."