An alert system to rank the threat level of coronavirus is set to be launched by the government.

Prime Minister Boris Johnson is expected to unveil the threat level system, which will use a scale of one to five, as part of a televised address at 19:00 BST.

He will also unveil a new slogan telling the public to "stay alert, control the virus, save lives".

Mr Johnson's address will also give an update on the UK's lockdown measures.

He is not expected to provide exact dates for when the restrictions - first announced on 23 March - might change.

The threat level system will apply to England only but the government is working with the devolved administrations as they develop their own.

It is understood the system - with alerts ranging from green (level one) to red (level five) - will be similar to the one used to keep the public informed about the terror threat level.

Mr Johnson is expected to say England is currently at stage four but moving towards stage three.

New slogan 'needs clarity'

Communities Secretary Robert Jenrick told the BBC's Andrew Marr Show it was the right time to "update and broaden" the message to the public from "stay at home" to "stay alert".

"Stay alert will mean stay alert by staying home as much as possible, but stay alert when you do go out by maintaining social distancing, washing your hands, respecting others in the workplace and the other settings that you'll go to," he said.

But shadow health secretary Jonathan Ashworth said the government must clarify what the new slogan means.

"When you're dealing with a public health crisis of this nature you need absolute clarity from government about what the advice is. There is no room for nuance," he told Marr.

"The problem with the new message is that many people will be puzzled by it," he added.

The alert tool - to be administered by a new "joint biosecurity centre" - will reflect the virus threat in different parts of the country, meaning the threat level in one city could differ quite widely from another.

This could inform the local alteration of restrictions in England.

A meeting of the government's Cobra emergency committee involving the cabinet, devolved nations and the mayor of London will be held before Mr Johnson's televised address on Sunday evening, with the plans to be put before Parliament on Monday.

Mr Jenrick said the UK government's "strong preference" was for the devolved nations "to move as one".

---

The UK government's new slogan is part of moving into the next phase of the response to coronavirus.

Staying at home where possible will remain part of the strategy, but ministers want to "broaden the message".

Some are worried the new campaign is ambiguous and muddies the water.

In Wales and Scotland, the devolved governments have made clear they will keep the original slogan - "stay at home, protect the NHS, save lives".

So from tomorrow, messaging will be different in different parts of the UK.

And I understand there are real concerns in the Scottish government about how people will react - with fears it will be harder to get the public to follow their advice to stay at home unless essential.

The communities secretary also told Marr a contact tracing app being trialled on the Isle of Wight had been downloaded by about 50,000 people.

Asked about reports a second app was being developed, Mr Jenrick said: "We are learning lessons from the other apps that exist elsewhere in the world and if we need to change our app, we will do. That's the point of piloting this before we take it national."

The prime minister will warn the nation that the UK is entering the most "dangerous" phase of the battle against the virus, according to the Sun on Sunday.

He told the paper: "Mountaineers always say that coming down from the peak is the most dangerous bit. That's when you're liable to be over-confident and make mistakes... you have to make sure you don't run too fast, lose control and stumble."

'Best protection is your front door'

Prof Jason Leitch, national clinical director for the Scottish government, told BBC Breakfast he "can't explain" what the prime minister's revised message of "stay alert" means.

Prof Leitch reiterated what First Minister Nicola Sturgeon said earlier this week - that the key message for Scotland "remains 'stay at home'" - and added: "I think the messaging is really important.... the best protection for this virus is your front door. There isn't any question about that."

On Saturday, Transport Secretary Grant Shapps warned the government would proceed with "extreme caution" when lifting lockdown measures.

Mr Ashworth said while the lockdown remains important, it is "fundamentally a blunt tool" with "huge societal problems" building the longer it continues.

Meanwhile, a model of protective goggles is being withdrawn from hospitals and care providers because they do not meet standards for coronavirus settings, the government has said.

A Department of Health and Social Care spokeswoman told the BBC it had issued an alert advising against the use of "Tiger Eye protectors" in clinical settings related to Covid-19.

The goggles were bought in 2009 and are now being removed from the supply chain, the spokeswoman added.

"The safety of health, care and all frontline staff is our top priority... we are arranging replacement stock for trusts who need immediate supply," she said.

Some 15 million goggles have been recalled, according to the Sunday Telegraph.

In other developments:

• The Department of Health has confirmed 50,000 coronavirus test samples were sent to the US earlier this week after problems in UK laboratories. It comes as the government failed to hit the 100,000 daily testing target for the seventh day running
• More than 70 public figures - including Baroness Doreen Lawrence and author Malorie Blackman - are calling for a full independent public inquiry into deaths from Covid-19 among people from ethnic minority backgrounds
• The Prince of Wales has hailed the "dedication, resilience and hard work" of Britain's postal workers during the pandemic. Charles left a letter addressed to "Everyone at Royal Mail" outside his home in Birkhall, Aberdeenshire
• Decisions on which shops reopen after lockdown should be based on safety, not their size or business type, a retail chief has told the BBC
• Religious leaders have warned that social distancing will be impossible in some places of worship if government ministers allow them to reopen - with one leader saying churches might not return to normal services before the end of the year
• Another 346 UK coronavirus deaths were recorded on Saturday, taking the total to 31,587