Covid: Why did Boris Johnson decide to lock down England?

We are used to events moving quickly in this pandemic, but how did we get from Boris Johnson urging parents to send their children back to school, to announcing a strict lockdown for England the next day?

This was not the plan. At least not at the start of Monday.

In the space of 36 hours, the government went from urging parents of primary pupils in England to send their children back to school - to shutting classrooms for almost all children for at least seven weeks and telling everyone to stay at home.

So what changed?

On Sunday morning, the prime minister made clear more restrictions were coming in England. He told the BBC's Andrew Marr the situation was tough and he believed the public was reconciled to stricter rules.

But there was little in the way of detail of what could be coming. Mr Johnson urged parents to send their children back to schools if they were open. Classrooms, he said, were safe. It was, he insisted, sensible to keep primaries open.

It was thought the next step would be putting more areas into tier four - and that some schools could be closed for a while in areas where the virus was spreading fast. But it seemed that a decision could be a couple of days away. A March-style "stay at home" lockdown was not being discussed widely.

On Monday morning, in another interview with broadcasters, the PM said the impact of the new tier four was "a bit unclear", suggesting the government still wanted time to assess the numbers (remember it was only introduced on 19 December).

There was no sign of the urgency which was to come - Mr Johnson said further restrictions would be announced "in due course".

Over the next few hours, the situation developed at a dizzying pace.

A key factor was the UK's four chief medical officers who, around the same time Boris Johnson was being interviewed at a hospital in London for TV, made the decision to move the whole of the UK to the highest Covid alert level.

They drafted a joint statement saying that the NHS was under "immense pressure" and that they were not confident it could handle a further sustained rise in cases.

It concluded: "Without further action there is a material risk of the NHS in several areas being overwhelmed over the next 21 days".

That put huge pressure on Boris Johnson to act ahead of the release of that statement.

Although the Covid alert level had been forgotten by many, according to the measures revealed in the summer it meant extreme strict social distancing should come back. The BBC revealed the decision a few hours later and it was officially confirmed at 1800 GMT. It was now hard to see how a lockdown could be avoided.

Downing Street has also made it clear the prime minister was influenced by new statistics he was shown on Monday which revealed more than 80,000 people had tested positive in the UK on 29 December - and that the new variant was spreading in all parts of England.

But Boris Johnson decided to act under significant political pressure too.

Scotland's first minister Nicola Sturgeon had met her cabinet on Monday morning - and they had signed off the decision to tell Scots to stay at home and to delay the re-opening of schools.

Ms Sturgeon announced that decision just after 1400 GMT - and said she was now more concerned than she had been at any stage since March. She argued Scotland was four weeks behind parts of the south of England, ramping up pressure for a stronger lockdown there too.

Labour was also calling for a national lockdown in England to be imposed immediately - telling the prime minister he had to clarify what was happening rather than "hinting" at further restrictions.

The former health secretary - Conservative MP Jeremy Hunt - added his voice to calls for urgent measures.

In the end, Downing Street announced plans for a televised national address at 1450 GMT - and a few hours later the prime minister revealed the new lockdown.

Ministers argue the PM made a decisive decision in the face of "new information" about the spread of the virus. Whitehall sources also say the insistence that schools were safe was right - and that closing them is about limiting movement in the community rather than stopping the spread in schools themselves.

But the question Boris Johnson has faced on a number of occasions now is: is he prepared to act quickly enough?