Britain's much-maligned multi-billion pound Covid-19 test-and-trace system has improved, but is still missing targets and the results of millions of tests to find asymptomatic cases have not been reported, parliament's spending watchdog said.

The programme, which was given a 22 billion pound ($30.6 billion) budget, was launched by Prime Minister Boris Johnson in May 2020 to oversee testing of those who thought they had the virus and then to trace the contacts of those who tested positive, according to Reuters.

The National Audit Office said the NHS Test and Trace Service had expanded its capacity and improved its performance since a report last year found it was failing to meet its objectives.

But, there were still serious problems. During a surge in cases in December, only 17 per cent of people received test results in 24 hours against a target of 90pc, the NAO said.