There is an "ambition" for all primary school children in England to spend a month back at school before the summer holidays, says the government's updated Covid-19 guidance.
But to support social distancing there will be class sizes of no more than 15 pupils, staggered break times and frequent hand washing.
The National Education Union rejected the reopening plans as "reckless".
Parents who choose to keep their children at home will not face fines.
Prime Minister Boris Johnson, in his televised address on Sunday, said if the level of infection remained low enough, children in Reception, Year 1 and Year 6 in primary schools might begin to return, from 1 June "at the earliest".
Further guidance on Monday announced plans to widen this to all primary year groups, including early years settings and childminders - but with strong warnings of delays if "insufficient progress is made in tackling the virus".
The decision to make Reception, Year 1 and Year 6 the first classes to return was "to ensure that the youngest children, and those preparing for the transition to secondary school, have maximum time with their teachers".
How will schools be kept safe?
Details set out by the Department for Education show classes will be divided into groups of no more than 15 pupils - and these small groups will not mix with other pupils during the school day.
The guidance says pupils should be kept two metres apart if possible - but it accepts that young children cannot always be expected to keep that distance apart, from each other or staff.
There will be staggered break and lunch times, and different times for starting and finishing the school day.
Children will be encouraged to wash their hands often, cleaning of rooms will be more frequent and schools will be encouraged to use outdoor spaces.
But the wearing of masks is not recommended, for either pupils or teachers.
This follows the pattern of a return of schools in Denmark, which has used a system of teaching children in small groups which are kept separate from each other and where there is regular hand washing.
What's happening in secondary school?
Secondary schools and further education colleges are likely to stay closed until September - apart from pupils with exams next year, who will get more help in addition to their current online lessons.
Schools and colleges are told to "prepare to begin some face-to-face contact with Year 10 and 12 pupils who have key exams next year".
But Labour's Shadow Education Secretary Rebecca Long-Bailey said the government needed to address teachers' "concerns, anxiety and confusion".
"There is no information about how social distancing will work in schools, how teaching and support staff, pupils and parents will be protected from the virus, how small class sizes will be achieved."
Will parents be fined if they don't send their children to school?
More than 400,000 people have signed an online petition urging the government to give parents a choice on whether they send their children back to school this term.
But it is understood that parents who decide to keep their children at home will not face fines for non-attendance.
At present, parents who are key workers have the option of sending their children to school, but there are no penalties for those who have not taken up places.
This temporary arrangement during the coronavirus crisis will also continue for the year groups going back to school.
"As a mum, I don't want to face serious repercussions for making a choice I feel affects the safety of my daughter during a global pandemic," said petition organiser, Lucy Browne.
How many children are likely to go back?
When schools were kept open for key workers' children, there were worries that too many would turn up. But in practice the opposite happened, with lower numbers than anticipated.
The latest guidance says 2% of pupils are currently going to school - and with no fines and lessons likely to be complicated by social distancing, it remains to be seen how many children will return.
But it will give more parents an option for going back to work - and children might be relieved to get out and see their friends again.
Speaking at the government's press conference on Monday evening, the chief medical officer for England, Prof Chris Whitty, said the risks of coronavirus were "very, vey low in children in contrast to other infectious diseases".
He said the question was whether re-opening primary schools would lead to a "significant upswing or a change in the R".
"The view is that if it's done very carefully, slowly, it is very unlikely to do that - but it has to be done very carefully.
"Teachers and parents are understandably concerned about individual risk and that's very much what we're consulting on at the moment with the profession, and it is very important that we have a proper debate around that, to make sure people understand that we can do many things to reduce the risk."
What are the teachers' unions saying?
"This timetable is reckless. This timetable is simply not safe," says Mary Bousted, co-leader of the biggest teachers' union, the National Education Union.
She said the reopening plans had "stoked teachers' anxiety and triggered real confusion because the announcement is inconsistent on the importance of social distancing and how or whether it can be achieved in schools".
"The profession has got very serious concerns about that announcement of 1 June - whether indeed it is possible to achieve it, but also how to achieve that in a way which is safe for pupils and staff," said Patrick Roach, leader of the NASUWT teachers' union.
Paul Whiteman, general secretary of the school leaders' union NAHT, said the ambition to return all primary pupils within the next seven weeks was "wildly optimistic, to the point of being irresponsible".
"School leaders do not want to see classrooms empty for a day longer than they need be - but there is not a school leader in the land who wants to risk admitting more pupils unless it is safe to do so."
What do parents think?
Many parents on the BBC's Family and Education Facebook page expressed concerns over safety.
Kirsty Smith said: "I have a Reception-aged child and then three others in years that aren't going back.
"First thing my youngest will want to do is give her teacher and friends a hug - she's five,
"Telling a child 'no this no that' is going to make them think they're constantly doing wrong by doing things that are natural to them.
"They may split the classes and stagger lunch times but they won't be able to sanitise every pencil, toy, lunch table even before the next child uses it."
Gurmeet Bhachu added: "No, my child is not going back to school until it is absolutely safe for them and teachers."
Louise Richards said: "Many children are in families with at risk members. They can't go back. It will simply disadvantage those and put the rest at risk."
Suzanne Mattinson said she would not send her child back to school until there was a vaccine.
"If my hand is forced, I'll remove him formally and home school."
What's happening elsewhere in the UK?
In Wales, the First Minister Mark Drakeford has said: "We're not going to be reopening schools in Wales in the next three weeks, or indeed in June," he said.
In Scotland, it seems unlikely that schools will re-open before the summer holidays, with most schools due to break up before the end of June.
In Northern Ireland, Education Minister Peter Weir has spoken of a possible phased return of schools in September.