Hundreds of schools in north west England have been 'strongly advised' not reopen on Monday morning after it was revealed the R rate has risen in the area.
Health officials at Blackburn and Darwen Council, which runs 85 schools in Lancashire, emailed local schools on Friday evening advising them not to reopen.
The same advice was given by public health officials in Tameside, Greater Manchester, to delay reopening for pupils other than vulnerable children and those of key workers, to June 22.
Hundreds of primary schools in the north west of England have been 'strongly advised' not to reopen on Monday after a rise in the R rate
It comes after new data showed the virus' reproductive rate, known as the R value, is higher than the crucial threshold of 1 in the north west region and Health Secretary Matt Hancock admitted there is a 'challenge' there.
The R value refers to the average number of people that will contract coronavirus from an infected person.
If it is 1 or higher, the virus will spread exponentially through the population, while a value less than 1 indicates the virus is in decline.
Officials from Queen Elizabeth Grammar School in Blackburn (QEGS) tweeted that they would not re-open until June 22 after latest guidance from the council to all schools across the borough.
QEGS headteacher Claire Gammon wrote a statement on the school website to parents and carers.
It said: 'Following updated guidance from Blackburn with Darwen Council this evening we are delaying the phased reopening of school until June 22.
'We appreciate this is late notice however we are having to work with an ever-evolving situation and flexibility is required by our whole QEGS community.'
Other schools in the borough, which has a population of 150,000, have also said teachers were 'devastated' after working 'tirelessly' to prepare for Monday.
Health Secretary Matt Hancock acknowledged that there was a 'challenge' in the area following the new statistics
Headteachers in Tameside were also told on Friday to delay the wider reopening of schools beyond key workers' children and vulnerable pupils 'until there is further assurance,' the council's director of public health, Dr Jeanelle de Gruchy, said in a letter to schools.
The north west has the highest rate of Covid-19 infections in England according to Public Health England (PHE), and data released on Friday afternoon suggested the R value is higher than 1 in the region.
Data from PHE released on Friday gave an R value of 1.01 for the North West and 1.0 for the South West, with all other regions below 1.
The Government has suggested a strategy of 'local lockdown' measures to fight any flare-up of the virus in particular areas.
However Andy Burnham, Mayor of Greater Manchester, has questioned whether such measures are workable, calling them a 'recipe for chaos'.
Both he and Liverpool City Region Metro Mayor Steve Rotheram have questioned whether lockdown relaxation was being lifted too soon, having been driven from London, with the regions and the North in particular not being listened to.
Council bosses in Liverpool and Gateshead led a northern backlash against the government announcement last month advising schools to reopen for Years 1 and 6 from June 1.
Health Secretary Matt Hancock, speaking at the Downing Street briefing on Friday, said experts on the Scientific Advisory Group for Emergencies (Sage) believe the UK's overall R number is below 1, but added that local lockdowns would be used when outbreaks are spotted.
Mayor of Greater Manchester, Andy Burnham, questioned whether local lockdown measures were workable
Liverpool City Region Metro Mayor Steve Rotheram joined Burnham in asking whether lockdown relaxation was being lifted too soon
He said the PHE data needed to be looked at 'in the round' with other data.
Mr Hancock added: 'The discussion of the higher R in the North West and the South West that's estimated compared to the rest of the country is an important part of moving towards a more localised approach rather than a national approach to the lockdown.'
In her letter to headteachers, Dr de Gruchy said that after initially supporting the gradual reopening of school in Tameside, the advice had now changed.
'Because of this change in R, and despite the excellent work undertaken, I am therefore strongly advising all schools and childcare settings to delay wider opening until at least June 22 for us to be more assured that the rate of infection is reducing and R is firmly below 1,' she said.
Colin Cox, director of public health in Cumbria, also warned on Friday that people should not be 'complacent' and lockdown restrictions could be tightened if the R number increases.