Small social gatherings will be permitted from Monday, after Boris Johnson announced an easing of lockdown restrictions – but also warned people they must not let their guard down.

The Prime Minister said groups of up to six people from outside one household can meet in private gardens, “provided those from different households continue strictly to stick to social distancing rules” by staying two metres apart.

He said groups can meet on private and public land for events such as barbecues, but they would not be allowed to stay overnight, or go indoors other than to gain access to a garden, he said.

The move will be a welcome boost for families and friendship groups, who until now have only been able to meet with one other person outside their household, in public spaces such as parks, while obeying social-distancing rules.

But Professor Chris Whitty, chief medical officer for England, told anyone planning such gatherings it was “absolutely critical” to maintain strong hygiene standards, including washing hands and keeping to social distancing rules.

Speaking at the daily Downing Street press briefing, he said: “If you were to do something like a barbecue, remember that passing things from one person to another, if you haven’t washed your hands you can transmit the virus that way.”

On social distancing he added: “If people are meeting in these new, slightly relaxed social distancing guidelines in terms of meeting outdoors, it is essential that people maintain two metres, and that is really important.”

Summing up, Mr Johnson said: “If you are going to do things like this you have got to be scrupulous and careful, otherwise you risk transmitting that disease again.”

Mr Johnson also urged people to avoid “seeing too many people from too many households” in quick succession, but said further guidance would follow.

He also said those who were shielding for health reasons should continue to do so.

The PM said he wanted people to be able to see friends and family and enjoy events like barbecues.

He said: “I know the toll the lockdown has taken on families and friends who have been unable to see each other.

“We will now allow people to meet in gardens and other outdoor spaces.

“These changes, that friends and families can now meet their loved ones, perhaps seeing both parents at once, or both grandparents at once, and I know for many people this will be a long-awaited joyful moment.

“But I must stress, that to control the virus, everyone must stay alert, act responsibly, strictly observe social distancing rules and stay two metres apart from those you do not live with.”

He added: “It’s very, very important that people understand the really limited nature of what we’re saying.

“We do want people to be able to see their friends and family, we do want people to see two grandparents at once but it’s got to be socially distanced, there’s got to be a maximum of six people.”

He added: “We don’t want people to stay overnight, we don’t want people to go to other households and stay there, I’m just afraid we’re not at that stage.

“What you certainly can imagine is there could be meetings of families in a garden, you could even have a barbecue provided you did it in a socially distanced way, provided everyone washes their hands, provided everybody exercises common sense.”