NHS England has declared coronavirus a level four incident - the highest level of emergency preparedness planning.

It comes as confirmed cases in the UK rose to 51 and Boris Johnson unveiled his plan for dealing with the outbreak.

Under the level four alert, all hospitals in England have been told to "assume that they will need to look after COVID-19 cases in due course".

A national incident management team and coordination centre have been set up for the coronavirus.

NHS regions must report centrally and set up their own incident teams, including having a 24/7 contact for "patient management, alerts, referrals and tracking".

Everyone in intensive care with a respiratory infection must also now be tested, as should everyone in a Severe Respiratory Failure centre.

The guidance says it is "now appropriate" to put some patients in "wider infectious disease units" - rather than specialist COVID-19 units - and they could be grouped in "all acute units" if cases continue to rise.

An NHS emergency preparedness adviser, who did not want to be identified, explained: "Level one is a localised incident, like a small fire, where the NHS trust can manage by themselves without any intervention.

"Level two is a larger incident, like a small flood, where the commissioners would have to get involved."

The former emergency department nurse, who was heavily involved in helping the NHS to cope during the 2009 swine flu pandemic, added: "A level three is declared when there is a regional emergency and level four, the highest emergency level, is declared when there is a national medical crisis."

COVID-19: Who is most vulnerable?

Twelve new UK cases were identified on Tuesday: eight had travelled from Italy, one from Germany, one from Singapore, one from Japan and one from Iran.

They are from London, Hampshire, Northamptonshire, Bury, Wirral, Greater Manchester, Humberside and Kent.

Health Secretary Matt Hancock said the NHS had been "preparing for a pandemic virus for over a decade" and was still in the containment phase.

But he said if global cases continue rising - especially in Europe - "we may not be able to contain the virus indefinitely".

Shadow health secretary Jonathan Ashworth called for assurances that workers not entitled to sick pay would not be forced to choose between self-isolation and earning a living if they get sick.

Prime Minister Boris Johnson has detailed the government's plan to deal with the virus.

He said police could be reduced to just dealing with very serious crimes and maintaining public order, while the NHS could be closed to all but critical care.

Emergency services all have measures in place to "fulfil critical functions" but they would have to reduce services should large numbers of staff become ill.

The government also said plans were in place to draft in the Army, if necessary, to maintain public order.

Mr Johnson said there were "long-established plans by which the police will, obviously, keep the public safe but they will prioritise those things that they have to do".

Coronavirus quarantine: What happens now?

He added: "And the Army is of course always ready to back-fill as and when, but that is under the reasonable worst-case scenario."

The 27-page plan also warned of a depletion in workforces across the UK and said one in five workers could be absent when the virus peaks.

The government said it would consider closing schools and universities, encourage working from home and a reduction in large gatherings.

Key points:

Police would "concentrate on responding to serious crimes" if they lose a "significant" amount of staff to illness

UK has stockpiles of medicines for the NHS, along with protective clothing and equipment for medical staff

If coronavirus becomes widespread, there will be a focus on essential services for those "most at risk"

The Ministry of Defence will provide support as needed

There will be increased government communication with parliament, the public and the media

Social distancing strategies could be implemented, which would include school closures, home-working, and reducing the number of large scale gatherings

Non-urgent operations and procedures could be cancelled and hospital discharges monitored to free up beds

Measures would come into place to help businesses with short-term cash flow problems

A distribution strategy for sending out key medicines and equipment to NHS and social care patients

Image: The number of cases in the UK stands at 51

Speaking at a news conference, the government's chief scientific adviser Sir Patrick Vallance, said restricting travel once the epidemic was "everywhere" would make "no difference at all".

He also said there was "no reason" for people to stockpile food and other goods.

Mr Johnson told reporters he continued to shake hands with people, adding: "I was at a hospital the other night where I think there were coronavirus patients and I was shaking hands with everybody, you will be pleased to know, and I continue to shake hands.

"People must make up their own minds but I think the scientific evidence is... our judgement is that washing your hands is the crucial thing."

The government plan outlines its response in four stages.

They include containing the outbreak, delaying its spread, mitigating the impact of the disease once it becomes established, and implementing a research programme aimed at improving diagnostics and treatment.

Officials are hoping to delay the peak of the outbreak until the spring and summer months when health services are less busy.

:: Listen to the Daily podcast on Apple Podcasts, Google Podcasts, Spotify, Spreaker

Legislation allowing the government to use extra powers to help control COVID-19 is expected to be passed by the end of the month.

Chancellor Rishi Sunak has also pledged more funds to fight the virus and has promised more cash in next week's budget.

Meanwhile, there have been concerns over the viability of events and large gatherings, including the London Marathon just eight weeks away.

The health secretary told MPs that "reacting too early or over-reacting carries its own risk", saying that the government would, therefore, "seek to minimise social disruption".

Image: Concerns have been raised about the virus spreading on transport networks

But Mr Hancock admitted some of the action would be "uncomfortable" but insisted the government was "quite prepared to do that if it's necessary".

However, speaking to Sky News' Kay Burley@Breakfast show earlier, he said the government was seeking "as targeted as an approach as possible" with the focus on elderly and others likely to suffer most from the virus.

There are currently more than 90,000 cases of coronavirus around the world and more than 3,000 deaths.

Virus Outbreak: Global Emergency - Watch a special Sky News programme on coronavirus at 6pm weekdays.