New government data has suggested that millions of coronavirus tests are not being recorded.
The Department of Health and Social Care (DHSC) revealed that since testing started, a total of 10.5 million tests have been "made available" but only eight million of those have been "processed".
This means as many as a fifth of tests – more than two million – are either not being sent back to laboratories or are being voided.
Number 10 accepted that some people were opting not to carry out the process of sending their test back for examination after receiving their home coronavirus testing kit.
The Prime Minister's official spokesman told reporters: "It will be the case that some members of the public may order a test and then for whatever reason they choose not to return that test.
"Obviously, if people are ordering tests it is entirely right that we should provide them with one."
The spokesman said he had not seen a "verified number on this" when the DHSC figures were put to him.
It comes as the UK Government announced changes to the way it will publish daily testing data.
Downing Street said the decision to stop revealing how many people had been tested every day was because the statistic did not take into account someone who had been tested multiple times since the outbreak started.
The figure has not been made public for several weeks.
The PM's spokesman said: "DHSC will no longer publish the number of people tested daily anymore and will instead publish the number of daily tests processed.
"This is because the daily people tested statistic only counts new people being tested.
"For example, someone who is tested in February and then tested again this month will only be counted once.
"Considering hospital and care home staff are now being tested on a regular basis, we don't think this statistic would be an accurate reflection of the amount of daily testing that is taking place.
"Test and trace statistics published weekly will still include the number of people who have been tested."
Number 10 said that, in the 24 hours up to 9am on Sunday, a total of 164,849 tests had been provided and the testing capacity for the day stood at 294,258.