Whole schools or entire year groups could be ordered to self-isolate if coronavirus outbreaks are detected once pupils return to class in England in September.

Schools in England have been told to keep children in class or year-sized “bubbles” and avoid creating “busy corridors” when all pupils return in an effort to limit the spread of the virus.

Government guidance on how to get all children back after the summer break – following up to six months at home – suggests older pupils should be encouraged to be kept away from other groups of students and staff.

It warns that health protection teams could order the whole school, or all pupils in a year group, to self-isolate at home if schools have two or more confirmed coronavirus cases within a fortnight.

Education Secretary Gavin Williamson announced the plans for getting all pupils back on the same day schools in Leicester closed as part of the city’s lockdown extension.

He told the Downing Street press conference that the Government would “have to take very specific action” if schools fail to reopen in September.

Mr Williamson also warned that every child will be expected to attend school in the autumn unless there is a “proper medical reason” for their absence.

Limits on group sizes will be also lifted in nurseries and childminders in England from July 20 so more children can attend in the summer holidays.

Addressing the House of Commons, Mr Williamson said: “It is critical to ensure that no child loses more time in education and that from September all children who can be at school, are at school.”

He told MPs that the Government will provide all schools and colleges with a small number of home testing kits by the start of the autumn term.

The official guidance says mobile units can be dispatched to schools to test anyone who has been in contact with the child, or member of staff, who has tested positive.

Testing will focus on the person’s class, followed by their year group, then the whole school if necessary, the advice says.

Addressing the Downing Street press conference, deputy chief medical officer Dr Jenny Harries said the outbreak in Leicester was not caused by the return of schools, saying it was “community transmission”.

She urged parents to “control their teenagers” outside of school to stop the spread of coronavirus, adding that school is “quite a controlled environment”.

Mr Williamson said he hoped the Government’s “decisive action” taken in Leicester will avoid another “national shutdown” of schools in the future.

Schools have been told to avoid large gatherings, such as assemblies, and to avoid singing in larger groups, such as school choirs and ensembles.

Schools have also been advised to stagger break times, start and finish times, and to consider using “walking buses” to reduce the use of public transport.

Mr Williamson has suggested that school buses could also be segregated to ensure pupils remain in their “bubbles” to limit the spread of coronavirus.

On Thursday, exams regulator Ofqual announced plans to delay the start of next year’s GCSE exams to allow more time for teaching amid the disruption.

A headteachers’ union has warned that it will be “enormously challenging” for schools to keep children apart in year-group-sized “bubbles” in September.

Geoff Barton, general secretary of the Association of School and College Leaders (ASCL), said: “The logistics of keeping apart many different ‘bubbles’ of children in a full school, including whole year groups comprising hundreds of pupils, is mind-boggling.”

In other developments:

– Official figures showed 74% of people who have taken a home test for Covid-19 have waited longer than 48 hours for the result.

– NHS Test and Trace figures showed 27,125 people who tested positive for Covid-19 had their case transferred to the system in its first four weeks of operation but just 74% of them were reached and asked to provide details of recent contacts.

– Some 132,525 people identified as recent close contacts of people who had tested positive for Covid-19 were reached through the tracing system and asked to self-isolate, but 20,918 other contacts were not reached.

– Around 25,000 people in England – 0.04% of the population – had Covid-19 at any given time between June 14 and June 27, according to Office for National Statistics estimates.

– Plans to draw up a list of countries to be exempted from quarantine requirements have become the subject of a dispute between the Governments in Westminster and Holyrood, with Scotland’s Justice Secretary Humza Yousaf claiming there had been a lack of consultation with the devolved administrations.

– In Scotland, it will be mandatory to wear face coverings in shops from July 10.

– In Wales, pubs and restaurants will be able to open from July 13.

Ahead of Saturday’s reopening of pubs in England, Downing Street urged people not to “overdo it” as they sampled the taste of freedom.

Boris Johnson will use a Number 10 press conference on Friday to urge the public to behave sensibly.

“It is hugely important that everybody follows the advice and makes sure that they don’t overdo it,” the Prime Minister’s official spokesman said.

Health Secretary Matt Hancock said: “This weekend, enjoy the relaxations, but please enjoy them safely to continue to break the chain of infections.”