Weather forecasters have predicted overnight temperatures as low as -13C this week as a 'polar plume' from the North Pole hits Britain.
Overnight temperatures are expected to plunge below freezing in northern England, Scotland and Northern Ireland.
Netweather says: "We've had to break out the snow icons again for the northern hills and mountains at times during the week ahead, and Monday morning is looking cold and wintry, with a risk of snow over high ground for much of the northern half of the UK and Ireland.
"The frosty nights continue too, so if you have put any you plants out, you'll need to protect them through the next week."
Southern areas could see floods as heavy rain hits. The north is likely to experience hail, sleet and snow, the Daily Telegraph reports.
Leon Brown at The Weather Channel says: "Sunshine and showers with snow are expected over northern hills.
"The cool theme continues to early May with wetter weather over the south and drier in the North with sunshine and some wintry showers."
Meanwhile, Steven Keates, from the Met Office, tells the Daily Telegraph: "It's going to be a bit of a shock to the system.
"The whole week is going to be really quite cold and strong winds will only accentuate the effect."
Speaking to the Daily Mirror, Met Office forecaster Greg Dewhurst said: "Frost will be quite widespread in northern rural parts on Monday morning.
"Wintry showers are possible in northern England and Scotland until Tuesday, with low pressure bringing rain to all on Wednesday."
Mr Dewhurst adds: "It will turn cooler again in a northerly on Thursday and the Bank Holiday weekend is likely to stay cool and unsettled, with gales in Scotland and northern England at times."