Some Covid test providers have stopped selling tests over the Christmas period - as demand mounts.
Two firms that were previously among the most reliable in Which? Travel research have told us they now can't guarantee test delivery in the next few weeks, due to postage limitations.
The government recently changed the rules on day two tests, requiring all travellers returning to the UK to take PCR tests, rather than cheaper, quicker lateral flow tests - regardless of vaccination status.
Most day two travel tests are sent to the laboratory via Royal Mail priority mailboxes, but Royal Mail does not process collections on eight separate days between Christmas and Tuesday 4 January, meaning labs are predicting delays in samples being returned.
In other words, if you post a private Covid test via Royal Mail on Christmas Eve it won't even be received by the laboratory until 30 December at the earliest.
If you post it on New Year's Eve it won't be received by the laboratory until 5 January - or 6 January in Scotland.
The issue of delayed kits and results is likely to be worsened by the fact that many laboratories are also struggling to deal with demand, or are pausing the service because of postal issues.
The in our recent research, Source Bioscience, has warned: 'We are experiencing extremely high levels of demand for our Covid-19 testing services. Your test of choice may not be available at the moment.'
It's since said that it won't be providing testing direct to the public at all over the Christmas period, as it can't guarantee that people will get results in good time.
The second-best provider in our research, Testing For All, has said: 'Please note we will pause sales for our PCR Day 2 from 19 December until 4 January 2022 due to the expected performance of the postal network used to return samples to the lab.'
Unfortunately this means customers are likely to be forced to use higher-cost providers or take expensive tests at the airport or in a clinic.
It is still possible to buy Source Bioscience tests from firms on the gov.uk list - but these can cost up to £89. This is because Source Bioscience, as a laboratory, is continuing to operate over the Christmas period selling tests to business clients, who sell them on at a premium to the public.
Late fit to fly test results: Some countries, such as Portugal, currently require a fit to fly test before allowing UK citizens to enter. If you're unable to take the test on time, you won't be allowed to board your flight / Eurostar.
Late day two tests: Almost everyone, regardless of vaccination status, is now expected to take a day two PCR test when returning to the UK, from any green list country. You'll need to self-isolate until you get your negative result, meaning it's crucial you can receive your test and get the sample to the lab in a timely manner. If not, you'll need to isolate for longer.
Book in advance where possible Test availability may be limited, and you can book your day two test as soon as you've booked your trip. Booking a test as soon as you know your travel dates will help make sure your day two test is waiting for you when you return.
Choose a company that doesn't rely on Royal Mail If you need a fit to fly PCR test for departure to a country that requires them then you should avoid using Royal Mail delivery over Christmas. Some providers link to their own courier services but this is likely to be much more expensive.
Check whether you can collect/drop-off in person If you live near a laboratory you may be able to send your sample by courier or deliver it in person - but check which options are available to you with your chosen lab before attempting alternative delivery options. The laboratory that Testing For All uses, Salient Bio, is in south London and fit to fly tests can be dropped off there.
Consider taking a test at the airport It's possible to take day two tests when you arrive at the airport. Collinson tests can currently be for £69 at many airports (£55.20 with a discount code from some airlines), or £99 for same-day results (£79.20 with a discount code). Other firms also provide tests at airports. Taking the test in-person cuts out waiting for a kit to arrive, and for samples to return to the lab.
There are currently 20 firms quoting tests for 'self swab at home' for less than £30 - some as little as £15 - but none will deliver for that price. In some cases the price including delivery can be up to £90.
If you want a test to be delivered to your home, we haven't seen any cheaper than the £35 offered by Source Bioscience, but that's currently not available.
The cheapest tests for delivery are typically those ordered direct from the major laboratories:
Biograd Diagnostics (£40)
Nationwide Pathology (£40)
Randox £48 (or £43 with voucher from your airline).
Although Which? Travel has had concerns with some of these firms, travellers who don't want to pay extra for a courier or in-clinic test might not have much choice. With the current Royal Mail delays there is still a risk of your test or result being severely delayed.
Randox tests are returned through its own network of drop boxes and it told us that it hoped to maintain the same level of service over Christmas. If you use Randox, it's important to check that you can easily find a drop box near you.
It's likely that the cheaper test providers will see high levels of demand so another option might be to choose a slightly more expensive test.
C19testing charges a premium - £64 - but does have the best refund policy of the firms we've looked at and very positive coverage on review sites and social media. but does say that it's open 24/7 over the Christmas period.
Do not use tests provided by firms that are not on the gov.uk list, as there's no guarantee that they meet the minimum standards.
If you book an NHS test and return it via a Royal Mail priority mailbox it can be collected and processed every day except Christmas Day or New Year's Day.
However, the private travel tests you'll need to take on day two after your arrival in the UK will not be processed on most days between Christmas and New Year.
If you post your sample late on Christmas Eve the laboratory won't even get hold of it for at least another six days.
We have asked UKHSA (UK Health Security Agency) if this will affect the reliability of any testing. If test samples are 'void' you will have to take another test, causing further delays. Not all test providers will give you a new test for free.
The Royal Mail told us: 'Royal Mail delivers and collects mail six days a week, apart from on bank and public holidays.
'Royal Mail will resume normal service on 29 December this year, in line with the Christmas Day and Boxing Day bank holidays. This has been communicated to all of our account customers, including private test kit companies, in advance.