The NHS Covid-19 phone app will incorrectly tell people to self-isolate for longer than they should next week, the government has admitted.
From Monday, people who are found to have come into close contact with a positive coronavirus case will only have to quarantine for 10 days rather than 14.
But the Department for Health and Social Care (DHSC) said this change would not be reflected on the app until Thursday.
This is due to the time taken to develop and test technical changes.
It means people will be told to isolate for longer than necessary under the new rules.
Until the app issues the correct instructions, people in England have to follow the instructions given by human contact tracers if phoned by NHS Test and Trace.
If someone has been advised to isolate by the app, and not by NHS Test and Trace, then they will be allowed to leave isolation if their isolation countdown timer says before Thursday that three days remain. But from Thursday, the app will display the correct number of days, so you should follow its instructions precisely.
The chief medical officers of England, Scotland, Wales and Northern Ireland announced the reduction in isolation time in a joint statement on Friday.
“Self-isolation is essential to reducing the spread of Covid as it breaks the chains of transmission,” they said.
“After reviewing the evidence, we are now confident that we can reduce the number of days that contacts self-isolate from 14 days to 10 days.
“People who return from countries which are not on the travel corridor list should also self-isolate for 10 days instead of 14 days.
“People who test positive should continue to self-isolate for 10 days from onset of symptoms or 10 days from point of taking a positive test if asymptomatic.
“We urge everyone to self-isolate when appropriate. It will save lives.”
The delay is not the first time the app has experienced hiccups. In November it was revealed the wrong risk threshold was being used, resulting in thousands of people not being told to isolate.
The government was also forced to upgrade the app in October to reassure users worried by alert messages suggesting they had been exposed to the virus.
It had been sending alerts with the tag line “Possible Covid-19 Exposure” but with no follow up to inform people whether they had been exposed to the virus or not.