It is “crucial” that people isolate when identified as a close contact of someone who has tested positive for coronavirus, Downing Street has said.

A Number 10 spokeswoman said quarantining at home “remains the most important action people can take to stop the spread of the virus”.

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It comes after a minister suggested that isolating when pinged by the NHS COVID-19 app should be a matter for individuals and employers to decide.

Paul Scully, a business minister, told Times Radio: “It’s important to understand the rules. You have to legally isolate if you are on the… contacted by Test and Trace, or if you’re trying to claim isolation payments.

“The app is there to give… to allow you to make informed decisions. And I think by backing out of mandating a lot of things, we’re encouraging people to really get the data in their own hands to be able to make decisions on what’s best for them, whether they’re employer or an employee.”

But a Number 10 spokeswoman said: “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.”

This was reiterated by the prime minister’s spokesman, who told a regular Westminster briefing with journalists: “We want people, when contacted, either through NHS test and trace or through the app to isolate.

“It is an important tool that all of us can use, all of us can follow during this ongoing global pandemic.

“The minister did talk about the value that we’ve seen through the app in saving lives and breaking chains of transmission.”

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Meanwhile, The Times has reported that another business minister, Lord Grimstone of Boscobel, stressed in a letter to one large employer that the app was only an “advisory tool” and that people were not under any “legal duty”.

It is not a legal requirement to isolate if pinged by the NHS COVID-19 app, but official health service guidance has been that people should “self-isolate immediately” when told to.

There is a legal compulsion to isolate if contacted directly by Test and Trace, however, with the potential for fines for those who do not comply.

Latest figures show that a total of 520,194 alerts were sent to users of the NHS COVID-19 app in the week to 7 July, advising them to isolate, a record number.

There have been calls for the app’s sensitivity to be tweaked, amid warnings that isolation is wreaking havoc on certain industries by creating staff shortages.

But the government has ruled this out in recent days.

From 16 August, under 18s and people who are fully vaccinated will no longer be told to isolate if they come into close contact with someone who has tested positive.

Instead, they will be encouraged to take a test. Anyone who tests positive will still be legally required to isolate, regardless of their vaccine status.

The government has announced it will not be drawing up a list of fully vaccinated “critical workers” that will be exempt from isolating for work purposes.

Instead, employers will have to apply to government departments to exempt workers who are ‘pinged’.

Boris Johnson, who is himself isolating after being identified as a close contact of the COVID-positive Health Secretary Sajid Javid, told a news conference on Monday that isolation is “one of the only shots we have got left in our locker to stop the chain reaction of the spread of C”.

“I’m afraid that at this stage it’s simply a consequence of living with COVID and opening up when cases are high in the way that we are,” the prime minister said.