Weather forecasters have warned the UK to be prepared for “disruptive snow” and plummeting temperatures after an Arctic blast brought the coldest night of the season so far.

The Met Office said the early hours of Monday had been the chilliest night, with the lowest temperature, -7.8C, recorded at Tulloch Bridge in the Highlands.

By the morning, many households across Britain had to de-ice their car windscreens for the first time this winter after an unusually mild and dry start to the month.

This week is set to be strikingly cold. The UK Health Security Agency (UKHSA) issued the first amber cold weather health alert of the season and, along with the charity Age UK, warned that the conditions could be dangerous for vulnerable people, including the elderly.

The Met Office said up to 20cm (8in) of snow could accumulate in the worst-affected areas in the “first taste of winter”.

It issued several yellow warnings for the coming days, including rain, sleet and snow from 7pm on Monday to 10am on Tuesday, from Northumberland and Cumbria down to north Wales and the Midlands.

The most likely scenario, forecasters said, was for up to 20cm of snow to accumulate on hills above 300 metres and up to 10cm on hills above 200 metres.

“There is a chance of snow settling at lower levels, where 5-10cm would prove much more disruptive – this remains uncertain, but seems most likely across parts of Yorkshire and Derbyshire,” they added.

In the affected areas, there was a chance of power cuts, disruption to road and public transport and the risk of injury from slipping on ice. In Northern Ireland, a warning for snow and ice is in place from 3pm on Monday until 10am on Tuesday. It is likely to lead to difficult travel conditions and public transport delays.

In northern Scotland, a yellow warning for snow and ice stretches from 4pm on Monday until 10am on Wednesday. The Met Office spokesperson Nicola Maxey said that by Monday afternoon snow had mostly fallen on hilltops with 2cm falling in Lerwick, Shetland.

But she said more snow and ice is expected over the coming days, with temperatures plunging to below average levels for the time of year. “It is going to be quite a widely cold week – a few degrees below average both day and night for most of the country,” Maxey said.

It is predicted temperatures could drop to -2C in London on Friday, -4C in Birmingham and -7C further north.

“There’s likely to be a widespread frost overnight as we get this colder air coming in from the Arctic maritime air mass – cold air from the north pushing down across the country – which will be across the whole country by the middle of the week.”

Drivers have been advised to allow more time for journeys and to pack essential winter items such as warm clothing, food, water, a blanket, a torch, an ice scraper or de-icer, a warning triangle, a hi-vis vest and an in-car phone charger.

The Met Office has said travel disruption on Tuesday is likely. Merseyrail said it would be activating its cold weather plan which means the first service on all lines running without passengers to check conditions.