Watch Katie Rowlett's report
People are being urged to self-isolate if they have Covid-19 symptoms even if they have a negative PCR test result amid issues surrounding false negatives.
There have been a number of cases where people have tested positive using lateral flow tests - but then received a negative PCR result.
Trish Caller got a negative result for Covid-19 after using a PCR test - but she ended up being admitted to Musgrove Park Hospital with both Covid and pneumonia after her symptoms worsened.
'I’ve lost all my trust in the testing'
She is now recovering at home in Taunton - but says she has lost faith in the testing system.
Trish told ITV News West Country: "I’ve lost all my trust in the testing. Why don't these tests pick up [Covid]?
"It's Covid, it's what it's developed to do. I had Covid, 100 per cent."
Pamela Constable, from Shipham in Somerset, says she has also suffered as a result of the testing system.
She recently tested positive via multiple lateral flow tests at home but then received a negative PCR result.
The nursery worker was surprised by the discrepancy and booked herself a second PCR test before returning to work as a matter of precaution.
She said: "By the Monday morning my symptoms had got worse. I normally don't work on a Monday, so I didn't go into the nursery that day. I went for another PCR test and that was then positive."
She said she could have passed the virus on to other people had she accepted that first negative PCR result: "The normal protocol is if you have a positive lateral flow and then you get a negative PCR you carry on as usual.
"If I was to have done that, I would have taken that into a nursery with lots of children, staff and lots of parents.
"It is quite worrying, that you could easily pass that onto somebody else and not realise and you're trusting those tests.
"I'm also high risk as well. You could potentially be giving that to someone else that's considered high risk."
Impacts on education
The problem has severely impacted schools and education, including Katherine Lady Berkeley’s School in Gloucestershire, where more than 40 students and six staff members have been forced to self-isolate because of coronavirus.
Just under 3% of the school’s population are absent because of the virus – which is below the national average – but precautions are in place because staff and students have had positive lateral flows followed by negative PCR results.
Elsewhere, Matthew Evans, the headteacher at Farmor's School in Fairford, has called for clearer advice about self-isolation rules after experiencing significant disruption to the school calendar.
He said: "In the past couple of weeks, we think we've lost about 60 school days to positive lateral flow tests that have then come back with a negative PCR.
"And that's about the same as the number of days we've lost for genuine positive cases who've had to isolate for 10 days each.
"I'm really interested to know why this is happening. In the meantime, it will be useful to have more guidance as to how schools should respond and what we should be telling students and parents."
Watch headteacher Matthew Evans' interview below
Why are people getting false negatives?
Some academics say the tests aren't the problem and one explanation is that the people are not doing them correctly.
Dr Bharat Pankhania, an expert in infectious disease at the University of Exeter, believes people need to be more careful when swabbing their nose and throat during the tests.
He said: "For us to really get a good sample, equals a scrape.
"You really need to put that PCR swab deep inside, scrape and pick up cellular matter, which is then put into the PCR tube and then analysed.
"And unless you do a good scraping, a good sample, you are likely to pick up not much and therefore you get a false negative."
What are the rules around testing and self-isolation?
Susan Hopkins, Chief Medical Adviser at the UK Health Security Agency, said: "We have been made aware of some areas reporting positive LFD test results with subsequent negative PCR tests and we are looking into the cause.
"If you get a positive LFD test, it’s important to make sure that you then get a follow up PCR test to confirm you have Covid-19.
"If you have symptoms of Covid-19, self-isolate and take a PCR test. With any test it is important to carefully read and follow the instructions for use on the test kit so as to avoid any incorrect readings."