Laboratories handling the day 2 tests for dozens, if not hundreds, of firms can't initially distinguish between Delta or suspected Omicron variants when they receive the result.
The government has announced that 'all contacts of suspected Omicron cases must self-isolate, regardless of their age or vaccination status.' However, Randox and many other travel test firms use PCR tests that are not able to pick up indications of Omicron until they have carried out genome sequencing.
Their tests do not guarantee that the patient has Omicron rather than another variant - genome sequencing is needed to confirm it - but they do reveal 'suspected cases'.
Two other laboratories we contacted, Randox and Nonacus, told us that their tests can't initially detect suspected Omicron. Another laboratory, Expert Medicals, did not reply.
Most people entering the UK are now obliged to take a PCR test on or before day 2 after they arrive. One Covid test provider told us that the majority of private firms sell tests that are not able to detect the initial signs of Omicron.
The UK's biggest Covid test provider, Randox, is not able to detect suspected Omicron cases until it carries out genome sequencing, which it said takes between 40 and 60 hours. Its tests are also used by dozens of other firms on the official government list.
It told us: 'In order to confirm the difference between Omicron and Delta, the sample needs to be genome sequenced. Randox has high volume rapid sequencing capability on-site to determine the Omicron variant.'
Another major laboratory, Nonacus, used by Testing For All and dozens of others, also told us it can't initially spot the tell-tell absence of a gene that implies a higher risk that the variant is Omicron. It, too, said that it can confirm the variant with genome sequencing.
Expert Medicals, described as one of the UK's biggest providers until it was removed from the government list in the summer, previously used Oncologica tests as well as its own laboratory. Oncologica has since stopped working with it.
The UKHSA (UK Health Security Agency) said that contacts of people who've bought tests such as those offered by Oncologica, Source Bioscience or Nationwide Pathology could potentially be told to self-isolate more quickly, if they're suspected of having Omicron.
This is because, while only genome sequencing can confirm Omicron, test and trace will start calling close contacts of cases as soon as there is a suspicion of the new variant.
Suspicions would arise if the person with the positive test has just returned from a high risk country, if they caught the coronavirus from somebody known to have Omicron, or if the test used revealed a suspicion of Omicron.
The UKHSA also said that around 60% of the tests used in the UK are able to detect suspected Omicron cases - while 40% can't. That includes community testing and testing in hospitals and social care, as well as travel testing. For community testing alone, fewer than half of all test providers are able to spot signs of Omicron.
Government rules state laboratories have up to 120 hours - five days - from when they get the result to when genome sequencing is confirmed.
This would mean that contacts of people with Omicron might not be told that they have to self-isolate until a week after the test was taken.
However, UKHSA said that genome sequencing can be done much more quickly than this - often within 48-hours. However, that still means that suspected cases could be detected two days later by some firms than others.
Some laboratories use a test that targets three gene types - one of which is called the S-gene.
With the Omicron variant the S-gene isn't present. That can be a sign that the laboratory has discovered Omicron rather than Delta.
Other laboratories use dual-target tests, which detect two genes instead of three, so they wouldn't be able to tell immediately whether the S-gene is missing.
The absence of the S-gene is not a definitive sign of Omicron. Nonacus told us:
'Multiple SARS-CoV-2 variants including mu, beta, omicron and any new variant with S-gene mutations cause S-gene target failure and so it is not true that S-gene target failure can diagnose or confirm omicron variant alone.
The best and gold standard way of fully genotyping the virus is via whole genome sequencing which ourselves and other labs undertake alongside the more rapid PCR'.
The problem is that if the government wants contacts of all 'suspected' cases of Omicron to self-isolate, they need that to happen immediately - rather than wait until the variant is confirmed.
Even without problems of spotting new variants, many Covid test providers have failed to provide timely results throughout the pandemic.
Travellers obliged to take day 2 tests after arriving in the UK have complained of both tests and results arriving late, or not all, so confirmation could take much longer than a week in some cases. Some, such as
Under the current rules most people coming into the UK have to take a test on or before day 2 after they arrive in the country, and self-isolate until they get results. It's more important than ever to