Health officials have admitted publishing the wrong figures about the number of people testing positive for coronavirus in part of Northern Ireland.
Inaccurate and incomplete data on the number of confirmed cases in the Western Trust area was released by the Public Health Agency (PHA) this week.
It has now been corrected.
Since April 4, tests on people within the Western Trust have been carried out at laboratories in Altnagelvin hospital.
According to the PHA, the results of these tests were not in its daily updates "due to reporting issues".
The Belfast Telegraph understands these were because the Western Trust supplied data in a different format to other Trusts, but this has now been resolved.
Gary Middleton, a DUP MLA in the area, said providing the wrong information could be "disastrous".
SDLP MLA Mark H Durkan said: "People must continue to treat the coronavirus with the severity it requires. Misleading figures, due to omission, is unhelpful and causes unnecessary confusion."
The Belfast Telegraph has reported in recent days that the number of cases - and deaths - from Covid-19 are likely to be significantly higher than reported.
The PHA publishes the numbers of people who have tested positive for the virus from each council area.
Figures for the two main council areas served by the Western Trust - Derry and Strabane and Fermanagh and Omagh - had remained mostly static this week.
In Derry and Strabane, with a population of well over 150,500, the number of positive tests on Monday, April 6 as reported by the PHA was 51 and on Thursday, the figure was 52.
On Friday this spiked to 77 after all the tests results supplied by the Western Trust were used by the PHA.
Similarly, the number of positive tests in the Fermanagh and Omagh Council area, where the total population is almost 117,000, showed just one additional case from Monday, when the figure was 42 until Thursday.
Once the PHA included all data from the Western Trust, the figure rose to 52 on Friday.
As there is no community testing anywhere in Northern Ireland, the figure for the actual number of people who have contracted coronavirus is still a mystery.
The Chief Medical Officer, Dr Michael McBride, has previously said he believes there are "many thousands" of cases in Northern Ireland.
Dr Connor Bamford, a virologist at Queen's University, last week estimated the true number of cases could be over 50,000.
Mr Durkan, an MLA for Foyle, said getting accurate and information to the public from the PHA was crucial if complacency was to be avoided.
He said: "As elected representatives, we are constantly asking people to follow the Public Health advice and rightly so.
"However, it is absolutely crucial that this information is accurate and fully up to date.
"We need to ensure the information we put in the public domain is completely accurate. The omission of data from a large Trust could also skew the figures across the North."
DUP MLA Gary Middleton said relating accurate information to the public is one of the most effective ways of making sure people stay at home.
He said: "Failing to provide the public with an accurate picture on the number of people in any part of Northern Ireland with coronavirus could prove disastrous and could lead to the spread of the virus.
"It is when people see the true extent of the spread of the virus that they will take seriously the very necessary control measures that have been put in place."
A spokeswoman for the Western Trust said: "Between Friday April 3 and Wednesday April 8, 344 Covid-19 test results were reported by the Western Trust."
A spokeswoman for the PHA said: "The PHA has worked closely with the Western Health and Social Care Trust around some of the reporting issues they experienced with regards the number of coronavirus (COVID-19) tests being reported to the PHA surveillance team.
"The reporting issues related to the data platforms that were being used by the respective parties to report into the PHA Surveillance function.
"This has now been resolved and the PHA is assimilating the backlog of data to provide a fuller report.
"From Friday April 10, the test data for the Western Trust area will now represent a more complete record."
Earlier this week, the chair of the British Medical Authority in Northern Ireland, Dr Tom Black, said the number of deaths in Northern Ireland could be 20 or 30% above the official figures being released.