The Prime Minister fears a second wave of coronavirus cases could hit the UK within two weeks, a Downing Street source has said.
There have been signs that the number of infections in England has already stopped slowing and may already be rising.
And on Tuesday, Boris Johnson told reporters that an increase in cases in countries like Spain shows “signs of a second wave of coronavirus in Europe”.
The Prime Minister is “extremely concerned” by what is happening abroad, reports The Mirror.
A Downing Street source told the Mail Online on Tuesday evening: “The PM is extremely concerned by what he’s seeing abroad and fears we could be seeing the same thing here in a fortnight.
“People have got to realise we are still in the middle of a pandemic. He wants to go further on opening things up and getting people back to work, but he knows it’ll be his head on the block if things go wrong.”
It comes as the town of Oldham has seen tough coronavirus lockdown restrictions reimposed after a worrying spike in cases.
The warning comes after a spike in cases forced Mr Johnson to impose 14-day quarantine on thousands of Brits returning from Spain.
Spain recorded 8,207 new cases in the week to July 19 – up from 5,292 the week before.
And on a visit to Nottingham, Mr Johnson said Britons must not drop their guard.
“The most important thing is for everybody in all communities to heed the advice, to follow the advice, not to be spreading it accidentally and get it right down and we’ll be able to ease the restrictions across the country,” he said.
“But clearly we now face, I’m afraid, the threat of a second wave in other parts of Europe and we just have to be vigilant.”
He faced fury for enacting quarantine with just a few hours’ notice. And No10 has refused to let those who come back from Spain get Statutory Sick Pay – saying all travel carries a risk.
Spain’s PM last night blasted Britain’s “error” of advising against “non-essential” travel to the whole country, despite the Balearic islands having low rates of Covid-19.
Pedro Sanchez snapped: “In most of Spain, the incidence is very much inferior to even the numbers registered in the United Kingdom.”
But defending Britain’s actions, the PM said: “What we have to do is take swift and decisive action where we think that the risks are starting to bubble up again.
“And let’s be absolutely clear about what’s happening in Europe, amongst some of our European friends.
“I’m afraid you are starting to see in some places the signs of a second wave of the pandemic.
“And we all remember what happened last time – it’s absolutely vital therefore that we make the necessary preparations here in the UK as we are doing.”
Despite France, Germany and other nations being at risk of being added to the quarantine list, Mr Johnson stopped short of saying British families should not go abroad at all.
He said: “Obviously these are decisions for families, for individuals, about where they want to go.”
Lib Dem MP Layla Moran, chair of the All-Parliamentary Party Group on Coronavirus, said: “With growing signs of a second wave of this deadly pandemic in Europe, there is no time to waste in learning the lessons from the first.
“The government’s complacent approach earlier this year cost lives, we cannot afford for the same mistakes to be made again.
“Urgent action is needed to improve the Test and Trace system and put in place a clear strategy for dealing with a second spike.”
The Prime Minister accepted the UK has to be “vigilant” regarding the threat of a second wave of Covid-19.
Mr Johnson added: “Where you can do local easing, then of course you should, but we’ll see what the review says.
“I have every sympathy with the views of the (mayor of Leicester Peter Soulsby) and local MPs, but we have to prioritise public health and apply simple common sense, and I think people also understand that.
“We’ll do everything we can in these tough times for businesses, for families, for people whose mental health has been affected by the lockdown, to give them the support that they need.”
Elsewhere, tougher measures have been introduced in Oldham in Greater Manchester as health bosses try to slow the spread of local outbreaks.
The ramped up restrictions were introduced by the local council rather than by the government.
The borough – which is in a pilot testing people in high-risk jobs – saw a spike of more than 100 cases in a week.
Starting today, the borough’s 250,000 residents are being told they cannot have ‘social visitors’ to their home.
They must also keep two metres apart from friends and family when seeing them outside, reports the Manchester Evening News.
Outdoor meetings will once again be limited in size, in a return to earlier lockdown restrictions that had recently been eased in England.
And the local council is asking vulnerable and elderly people who have been shielding to continue to do so for another two weeks.
The shielding change kicks in in England from this Friday, July 31.