LONDON, June 25 (Reuters) - 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 on Friday. 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. Critics say it has wasted huge sums and failed in its primary objective of breaking the chain of transmission. The National Audit Office said the NHS Test and Trace Service (NHST&T) 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% of people received test results in 24 hours against a target of 90%, the NAO said. Since the government embarked on a mass testing programme to find asymptomatic cases in October, 691 million lateral flow test kits had been sent out in England, but the results of only 14% - 96 million - had been returned. The report also noted that while the government"s scientific advisers said for the system to be effective no more than 48 hours should elapse between a case being identified and their contacts isolating, between January and April the median time varied between 74 and 97 hours. The NAO said the programme, which had underspent its budget by 8.7 billion pounds, required public compliance to work, but this was "still low or variable" with only a minority of those with symptoms requesting a test or self-isolating. "Some pressing challenges need to be tackled if it is to achieve its objectives and deliver value for taxpayers, including understanding how many lateral flow devices are actually being used and increasing public compliance with testing and self-isolation," NAO head Gareth Davies said. A Department of Health spokesperson said the NAO had recognised improvements had been made. “The testing and tracing being delivered across the country is saving lives every single day and helping us send this virus into retreat by breaking chains of transmission and spotting outbreaks wherever they exist," the spokesperson said. The department said since its launch, the service had identified 3.4 million positive cases and told 7.1 million contacts to self-isolate. It said they were "finalising analysis" to understand why so few lateral flow test results were being reported. The opposition Labour Party said the report was damning. "The government has been told time and again that if we are going to bring down cases, it needs to ensure people can afford to self-isolate, but it has refused to listen," said its health spokesman Justin Madders.