Downing Street has insisted it is “crucial” for people to self-isolate after receiving an alert from the NHS Covid-19 app, contradicting a minister who suggested people could make an “informed decision” on whether to quarantine.

No 10 moved to combat the confusion on Tuesday, hours after business minister Paul Scully said it was a decision for individuals and employers whether they should isolate after a “ping” from the app.

Another minister in the business department, Lord Grimstone of Boscobel, stressed in a letter to one large employer that the app is only an “advisory tool” and that people are not under any “legal duty”, The Times reported.

Given the risk of having and spreading the virus when people have been in contact with someone with Covid, it is crucial people isolate when they are told to do so, either by NHS Test and Trace or by the NHS Covid app

Although it has never been a legal requirement to obey the app’s instructions, the official NHS guidance has been that people should “self-isolate immediately” when told to.

In a sign that Downing Street was scrambling to get its message back on track, a No 10 spokeswoman said: “Isolation remains the most important action people can take to stop the spread of the virus.

“Given the risk of having and spreading the virus when people have been in contact with someone with Covid, it is crucial people isolate when they are told to do so, either by NHS Test and Trace or by the NHS Covid app.

“Businesses should be supporting employees to isolate, they should not be encouraging them to break isolation.”

Earlier, Mr Scully had reiterated on Times Radio that people are only legally required to isolate if they are contacted by NHS Test and Trace.

Asked whether this meant people should or should not self-isolate if “pinged”, he said: “We want to encourage people to still use the app to be able to do the right thing, because we estimate it saves around 8,000 lives.”

However, he added that it was “up to individuals and employers”.

The Prime Minister’s official spokesman declined to criticise Mr Scully over the confusion and said he was “not aware” of anyone talking to the minister over the matter.

On Monday night, Boris Johnson had stressed the importance of self-isolation as “one of the only shots we have got left in our locker to stop the chain reaction of the spread of Covid” after he lifted most of England’s legal restrictions.

Professor Sir Jonathan Montgomery, who chaired the ethics advisory board for NHSX on its contact tracing app, told Times Radio the Government needs to give clearer guidance to people about what to do when told to self-isolate.

“When we had no protection the risk was the same for everybody. If that risk is now reduced because someone is double-vaccinated, it feels as though we need more sophisticated advice,” Sir Jonathan said.

“If we are visiting an elderly relative or a cancer patient then take the ping seriously, but if you are doing something relatively Covid-friendly, then maybe make a different decision.”

Professor John Edmunds, a member of the Scientific Advisory Group for Emergencies (Sage), which advises ministers, said: “Contact tracing and self-isolation play an important role in stopping cases getting out of control and preventing deaths.

“It’s important we maintain these measures as stringently as we can.”

Shadow health minister Justin Madders accused the Government of “making it up as they go along”.

“Ministers mix messages, change approach and water down proposals when the public and businesses need clarity and certainty,” the Labour MP said.