The solstice marks the astronomical start of summer - but rain and thunderstorms threaten to blight parts of Britain as the season kicks off.
The remnants of a tropical storm called Bill are expected to bring rain to Glastonbury, sparking fears the festival could turn into a mud bath.
Thousands of revellers flocked to Stonehenge this morning to see the sun rise on the longest day of the year, b ut parts of the north of Britain will have showers, and may be struck by thunderstorms, while rain will hit large swathes of the country tomorrow.
The weather is expected to brighten, but forecasters warned the tail end of Bill, which caused floods and deaths in the United States, will hit the UK next weekend.
Tens of thousands of revellers who will descend on Glastonbury to see headliners including Kanye West and Florence and the Machine could face a soggy weekend.
Met Office forecaster Simon Partridge said "it is not the best of starts to the week", but brighter, sunnier spells will arrive during the week.
He said: "Next weekend we have Glastonbury, and it does look as though it will be slightly more unsettled.
"There will be some rain around on Saturday. It doesn't look like a complete wash-out but it will be a damper day.
"It is the remnants of tropical storm Bill that is expected to hit over that weekend. It is a big question mark as to how far north or south it goes and as to how much rain we'll get.
"It looks like a dry build-up to Glastonbury so at least they are not starting with wet ground."
Mr Partridge said two bands of rain will hit Britain tomorrow, bringing up to 20 mm of rain and making it a "grey and miserable start for many".
He said: "We have a band of rain that is going to move across the south of the UK during the early hours of the morning so that by rush hour everywhere south of the M4 should be under a band of rain.
"It is patchy in places, but it will be for a number of hours and we can see quite a lot of rain over the course of the morning."
He said temperatures will be cool for this time of year, at around 19C (66F) in the South to 16C (60F) in the North.
The weather will brighten up as the week goes on, with the mercury reaching highs of 23C (72F) in the South on Wednesday and around 20C (68F) in the North.
It is expected to become more unsettled as the weekend arrives.