Boris Johnson has urged people not to move gatherings indoors if it begins to rain.
Speaking at the daily Downing Street conference, the prime minister said people must remain outdoors if they meet people from other households.
Ignoring these rules 'could undermine and reverse all the progress that we have made together', he said.
Since Monday, people from different households have been allowed to gather in groups of up to six outdoors, as long as social distancing measures continue to be observed.
People must remain at least two metres apart when they meet up in parks, open spaces, and gardens.
The forecast over the coming days threatens to bring rain, meaning people may be tempted to bring gatherings indoors.
During the press conference, the prime minister urged people not to be tempted to go inside if they are visiting relatives or friends.
He said: "As the weather threatens to take a turn for the worse. Some of you may be tempted to move the gatherings that you have been enjoying outdoors indoors, out of the rain.
"I really urge you - don't do that. We relaxed the rules on meeting outside for a very specific reason, because the evidence shows that the risks of transmission are much lower outdoors.
"The risks of passing on the virus are significantly higher indoors which is why gatherings inside other people's homes are still prohibited.
"Breaking these rules now could undermine and reverse all the progress that we have made together.
"I've no doubt that that won't happen. I think the British public will continue to show the same resolve in fighting the virus as they have throughout this outbreak," he said.
On Wednesday, the government announced that a further 359 people have died in the UK after testing positive for coronavirus.
The total number of people who have died in hospitals, care homes and the wider community with the virus has risen to 39,728.
