Britain will be hit by the first frosts of winter this weekend - with warnings of snow in the hills.

The freak cold snap has come weeks early, after the coldest August for 17 years. Temperatures could plummet to -1C (30F) at night - 12 degrees C below the seasonal average, forcing millions to switch the heating on.

The chilly conditions come before the official end of summer - the autumn equinox on September 23 - and will see Britons digging out their duvets to keep warm at night. Forecasters warned the Midlands and Wales would be worst hit and the cold snap is a headache for farmers still harvesting spring barley.

Positive Weather Solutions' senior forecaster Jonathan Powell said yesterday: "September has turned decidedly colder, with a definite night-time chill. The coldest places on Saturday night will be the Midlands and Wales, with temperatures falling as low as -1C and causing frost.

"There is the possibility of a slight covering of snow over hills - and with the coldest air situated south of Scotland, it's England which is most at risk.

"This is a precursor for a cold October with a running threat of frost."

Temperatures today will also be noticeably colder - just 12 or 13C (55F) across Britain compared with a normal 15 or 16C (61F).

Brian Gaze, of independent forecasters The Weather Outlook, said even parts of the South were at risk or frost. "This is a really cold snap," he said. "The last time there was a cold outbreak this widespread in September was 2003, when northern England was below freezing."