Only 260 testing sites have been set up out of the 2,000 planned to bring an end to pingdemic disruption, Downing Street has admitted.
Workers in a number of critical sectors can avoid isolation by getting tested regularly under pilots aimed at preventing mass staff shortages due to Covid-19.
But only around one in 10 of the sites are currently operational - with many not expected to be ready until after self isolation is axed for fully vaccinated people on August 16.
A spokesman for the PM said: "We now have over 260 of these sites providing testing and we are in the process of establishing 800 further.
"Following them we will be establishing a remaining 1,200 over the coming days.
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"As we said when we set these out, our intention is to get these up and running as quickly as possible so we can ensure the industries that require people to ensure that they provide the critical service they provide and continue to do so."
The spokesman insisted that isolation "allows us to break chains of transmission" when pressed on why daily testing wasn't more widely used.
The Government scrambled to announce the plans last week after Covid cases rocketed, prompting record numbers of people to get pinged by the NHS app.
Workers have to take an initial PCR test and then daily lateral flows if they are asked to isolate after coming into contact with a positive case.
They will only be allowed into work if "there would otherwise be a major detrimental impact on essential services".
Eligible workers include those in prisons, waste collection, defence, the food industry, transport, Border Force and police and fire services, who have been alerted by the NHS Covid-19 app or called by NHS Test and Trace.
Boris Johnson has insisted that August 16 date for ending self isolation rules is "nailed on" despite calls for it to be brought forward to mitigate disruption.
People are legally required to self-isolate if they test positive or are contacted by Test and Trace but alerts from the NHS app are only advisory.
From August 16, fully vaccinated people will not be required by law to take a Covid test if they are identified as a contact of a positive case but it will be strongly advised.
It comes as new figures show 689,313 alerts were sent to users of the NHS Covid-19 app last week telling them they had been in close contact with someone who had tested positive for coronavirus.
This compares to 618,903 people who were pinged by the app in the week up to July 14 - itself a rise of 19% on the previous week, when 520,194 alerts were sent.
Communities Secretary Robert Jenrick urged people to isolate if told to do so, saying there "isn't very long to go" until August 16.
He told the BBC: "I appreciate that it is a significant number of people and it can be frustrating but the app is doing what we asked of it."
