Boris Johnson failed to meet his target of having all coronavirus tests completed within 24 hours by the end of June, newly-released data shows.

There was only a minimal increase in the proportion of tests completed within the timeframe after the Prime Minister made his pledge, statistics from the Department of Health and Social Care (DHSC) suggested on Thursday.

In making the commitment to Conservative former health secretary Jeremy Hunt on June 3, Mr Johnson said that 90% of tests were already turned around in 24 hours and that he would increase that to all but those with “insuperable problems” by the end of the month.

This rose to 91% of in-person tests being completed within the timeframe at regional test sites between June 25 and July 1, according to DHSC data.

However, the department said the proportion of tests returned by the end of the following day increased to 97.5%.

Separate data from NHS England said 97% of the health service’s own labs had an average test turnaround time of under 24 hours, up from 90% last week. But officials could not immediately provide the proportion of tests completed within Mr Johnson’s target.

Baroness Harding, the executive chair of the NHS test and trace programme to stem the spread of Covid-19, said they remained “committed to continually improving” the programme.

Mr Johnson made the commitment in response to Mr Hunt, who chairs the Commons Health and Social Care Committee, during Prime Minister’s Questions amid concerns that not enough people were being tested for Covid-19.

He said: “I can undertake to him (Mr Hunt) now to get all tests turned around within 24 hours by the end of June except for difficulties with postal tests or insuperable problems like that.”

Questioned on the 24-hour pledge at a briefing with journalists on Thursday, the Prime Minister’s official spokesman said “significant progress” had been made but admitted “we need to do more”.

The missed target comes as Labour leader Sir Keir Starmer called on the Government to “open the books” on its testing figures so the public can have confidence in the system.

Sir Keir was commenting after Sky News reported that the number of people who have been tested for coronavirus in the UK might never be known due to inadequacies in data recording while officials looked to ramp-up testing numbers.

The news outlet quoted one senior official as saying that in its early months, both the testing system and the data produced by it were simply “not fit for purpose” as efforts were put into meeting Health Secretary Matt Hancock’s target of testing 100,000 people per day.

Only last month Sir David Norgrove, chair of the UK Statistics Authority, wrote to Mr Hancock for a second time to warn that “the figures (on testing) are still far from complete and comprehensible”.

Sir Keir, speaking during a visit to Harlow, Essex, said: “I’ve seen this report from Sky on testing and it’s very concerning. I think there was an internal audit.

“What I have said to the Government is: open the books.

“We know that there has been double counting, we know that some of the tests that have been sent out have never made it back and we haven’t had the number of people tested every day published for over a month.

“This has got to be cleared up. It’s so important for confidence – this isn’t just about the numbers.”

The Sky report claimed that pen and paper data records were still being kept as recently as mid-May, while the DHSC has acknowledged – following revisions it has already had to make to its published information – that numbers for July 7 are likely to have been “over-reported”.

The DHSC defended its reporting efforts, saying there was a “balance to be struck between quality and timeliness” in the production of new statistics.

The department said the majority of UK statistics are produced under a much longer timeframe, which is why these were “published as experimental statistics, meaning we expected revisions to occur as the programme matures”.

A spokesman said: “We rapidly built, from scratch, a large-scale testing programme and can now provide a test to anybody who needs one.

“Throughout the pandemic, we have been transparent about our response to coronavirus and are always looking to improve the data we publish, including the way we update testing statistics.”

The spokesman said the DHSC would “continue to work closely” with the Office for National Statistics and the Office for Statistics Regulation on its approach to publishing testing figures.

A UK Statistics Authority spokesman said: “Good evidence, trusted by the public, is essential to success in containing the virus.

“We welcome recent first steps in improving the trustworthiness and transparency in information available and will continue to monitor the development and use of these statistics to ensure they are relevant, timely and of high quality.”