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Two households in Wales will be able to form one "extended household" to meet indoors and stay overnight from next Monday, the Welsh Government has said.
People can only be in one extended household, which cannot be changed once arranged. Travel restrictions are due to be lifted the same day.
First Minister Mark Drakeford said it means grandparents "will be able to see and hold their grandchildren again".
It follows similar "support bubble" arrangements elsewhere in the UK.
The changes will mean people can have physical contact, exercise, cook and eat together, and also stay in each other's homes.
There is no limit to the number of people that can be in the two households.
But, under the rules, if anyone in an extended household develops symptoms, the entire household will need to self-isolate.
The Welsh Government has asked people to keep records to help with contact tracing if that happens.
Image copyright Getty Images Image caption Two households will be able meet indoors from MondayThe members of the extended household cannot be changed.
People shielding are also included but the first minister warned that if they join an extended household it "will increase their risk of being exposed" to coronavirus.
Mr Drakeford said: "Creating extended households will enable many families to be reunited for the first time since March.
"Grandparents will be able to see and hold their grandchildren again.
"It will help support many working parents with informal childcare over the summer months and it will also support those who are caring for others."
Image copyright Jan Maddox Image caption Jan Maddox in Newport has been apart from Nigel Swaby in Tamworth since the day before lockdown.'Game-changer for me'
Jan Maddox, 71, has been living alone in Newport since March and has not been able to see her partner Nigel Swaby, who lives in a village near Tamworth.
"Virtually for the last three months I've been walking up and down the road," she said.
"It's the same houses, its the same hedgerows, it's the same everything."
Ms Maddox said the changes will be a "game-changer" for her, and if the travel restrictions end on 6 July she said her partner would "pick me up and he's going to take me back to the Midlands".
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'Think about the risks'
Mr Drakeford said extended households means that in some cases "especially in larger families, this may mean making some difficult choices."
The first minister asked viewers of the daily Welsh Government press conference to "think about the risks".
"Think about the consequences - if anyone in the extended household becomes ill, everyone will have to isolate for 14 days," he said.
"For some people this will have a greater impact than for others, and needs to be thought about carefully."
The moves coincide with plans to end Wales' stay local rules, and follows the opening of non-essential retail last Monday.
Image copyright Getty Images Image caption The moves come into force next weekIf the changes result in any "flare ups" the Welsh Government will be able to "make a connection between cause and effect", Mr Drakeford said.
"We make the changes on the basis of advice and the advice is always that these steps are proportionate and shouldn't lead to a further acceleration in the spread of coronavirus."
The first minister said he did not have the "detail in front" of him when he was asked how it would impact people in houses of multiple occupation.
Mr Drakeford said further guidance will to be published later this week.
The Welsh Conservatives welcomed the news but questioned why it could not have been announced last Friday "to prevent another lost weekend".
Angela Burns, health spokeswoman, said: "So many families in Wales need such a boost after so long apart, and so if there is science behind why the decision could not have been announced on Friday, then the First Minister should put it into the public domain."