Boris Johnson is facing calls for clarity after placing the UK on a police-enforced lockdown with drastic new measures in the fight against the coronavirus outbreak.

The Prime Minister ordered people only to leave their homes under a list of "very limited purposes", banned public gatherings of more than two people and ordered the closure of non-essential shops.

But police chiefs warned of phone lines being inundated with calls on Monday night with questions about what movements are still permitted, while MPs also called for answers.

In an address to the nation from Downing Street, Mr Johnson ordered people to only leave their homes to shop for basic necessities "as infrequently as possible", and to only perform one form of exercise a day.

They can also seek medical help, provide care to a vulnerable person or travel to work if "absolutely necessary", under the measures to last until at least Easter Monday.

"That's all - these are the only reasons you should leave your home," he said.

"You should not be meeting friends. If your friends ask you to meet, you should say no. You should not be meeting family members who do not live in your home."

A failure to follow the rules could see police dispersing gatherings and imposing fines, which Government officials said would start at £30.

After the UK death toll hit 335, the PM ordered the immediate closure of non-essential stores including those selling electronics and clothing.

All public gatherings of more than two people - other than those they live with - will be barred, the PM said.

Other premises to join pubs and restaurants in being closed are libraries, playgrounds, outdoor gyms, places of worship and hotels.

Parks will remain open for exercise, but all social events including weddings and baptisms will be stopped. Funerals can continue.

Mr Johnson said the measures will be "under constant review" and will be considered for relaxation in three weeks if the evidence allows.

Politicians who had piled pressure on the PM to enforce strict measures amid fears people were disregarding social distancing advice largely welcomed his announcement.

But there were calls for answers to the public's concerns after the PM scrapped his daily press conference on Monday to announce the measures in a statement.

Policing enforcement measures introduced by Boris Johnson in response to the Covid-19 pandemic will be a "real challenge" for officers, a police federation has warned.

Ken Marsh, chairman of the Metropolitan Police Federation, which represents officers in London, pleaded with the public to adhere to the measures as he warned that harsher ones could be introduced.

Mr Marsh said enforcement will be difficult amid "large amounts of sickness" among officers in the capital.

"So it will be very, very challenging and very, very difficult for us with what's put in front of us," he said.

"But we don't actually know what is being put in front of us yet and we're going to be asked to disperse crowds, it's going to be a real, real challenge.

"We will be dealing with it, but I'm not sure we will have the resources to be able to see it through."

He said the Army could step in and support police if numbers fall due to illness or self-isolation.

Police will have powers to disperse gatherings after Mr Johnson announced a ban on meetings of more than two people aside from those who live together.

Asked if major crime is no longer a priority, Mr Marsh said that although officers will police the same way "up to a certain degree", the coronavirus crisis had "taken over everything".

"This is the biggest thing that's ever happened in my lifetime and anyone's lifetime, really, and we need to get on top of it," he told Sky News.

"It's not to say we won't be policing, so people can't behave in any way they want, because we will still be policing in exactly the same way, but you will see measures changing as this changes."

Asked if he would like the measures go further, Mr Marsh said it could "absolutely become more draconian towards the public" if advice is ignored.

"Hopefully from this day, well, if they don't listen then there will be tougher measures," he said.

"I don't doubt for one minute, because the only thing you are going to see is hundreds and hundreds of people dying.

"And we don't want that, the police don't want that, I'm sure the public don't want that. It could be their loved ones. So we've got to work together."

John Apter, national chairman of the Police Federation of England and Wales, said in a statement that "saving lives and protecting the public" is the number one priority.

"The practicalities of policing this lockdown will be challenging, but policing will do all it can to keep the public safe, but we need the public to support us," he said.

"I ask that the public heed the advice and stay at home unless absolutely necessary.

"This will allow police officers to concentrate on keeping the streets safe and deal with all the regular calls we receive.

"This is about saving lives and supporting our NHS. I ask that the public gives us their support in this time of crisis."

British Chamber of Commerce Director General Adam Marshall said: "Businesses must play their part to help limit the spread of coronavirus.

"As the UK goes into lockdown, ministers must be crystal-clear about which businesses can continue to operate, and those which must now shut their doors.

"The new restrictions make it all the more important that the massive package of financial support announced by ministers last week is delivered to firms and employees on the ground as quickly as possible."

Union heads have reiterated their concerns about what the pandemic means for employees, some of whom are facing unemployment as shops close for at least three weeks.

Union of Shop, Distributive and Allied Workers (Usdaw) general secretary Paddy Lillis said: "Our members in supermarkets have had a torrid time over the last few days due to panic buying.

"Tonight the Government has made it absolutely clear that people can only leave home to buy essential items. We hope that brings to an end the misery that shopworkers have endured as this crisis unfolded.

"We are aware that some employers are laying staff off and asking them to go without pay until the Government's Coronavirus Job Protection Scheme commences, potentially at the end of April.

"Low paid workers cannot wait this long without pay and we urge the Government to act urgently to protect the workforce and for employers to act sympathetically."

Unite general secretary Len McCluskey also said: "The message to employers is clear: be responsible, help workers be part and parcel of the essential efforts to support the nation. Keep workers and their families safe."