The UK is experiencing its warmest February day on record and some of the highest temperatures ever recorded in winter, according to forecasters.

Temperatures in Trawsgoed, Wales, reached 20.3C, (68.5F) on Monday morning and rose to 20.6C in the afternoon. The previous highest recorded temperature in February had been 19.7C in London in 1998.

It is the first time temperatures have exceed 20C during winter, according to a Met Office official. The weather starkly contrasts with the beginning of the month, when sub-zero temperatures were recorded across the country.

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The Met Office’s chief meteorologist, Steve Willington, emphasised that the gap was notable, saying: “By the weekend we could see temperatures almost 10 degrees higher than what we might normally expect at this time of year, so there will certainly be a spring-like feel in the air across the country.

“North-western parts of the UK will be wet and windy at times this week, but settled conditions will spread to all parts of the country by the weekend, bringing plenty of sunshine and unseasonably mild temperatures that could well compete with current February temperature records.”

Responding to the Met Office reports, the Green party MP, Caroline Lucas, said the warmer temperatures were concerning. She added: “I like spending an afternoon in the sunshine as much as anyone, but it’s impossible to shake the feeling that this isn’t right. After last year’s unprecedented global heatwave and last month’s deadly temperatures in Australia, our baseline perception of what’s normal is shifting.

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“We are living through climate breakdown and, instead of taking urgent action, ministers carry on as if nothing has changed. The government must wake up to this crisis, urgently declare a climate emergency and introduce a ‘Green New Deal’ to provide a green job to everyone who wants one and transform our economy to prevent climate catastrophe.”

A Met Office representative said that temperatures in Wales were affected by local geography, with southerly winds rising over the east of the Welsh mountains and warming while descending down the other side in west Wales.

The country is experiencing warmer temperatures in general this week. London reached 19.8C on Monday – also the highest temperature recorded in February since records began.

This time last year there was widespread snowfall across the country between late February and March and some areas of the country were hit by blizzards.

Bob Ward, of the centre for climate change economics and policy at the London School of Economics, said that while warmer temperatures are often welcomed, they can be deadly. “People may find it very pleasant to be in warmer temperatures but they must also remember that it’s the same trend that saw hundreds of people die last summer as a result of overheating.

“Over the last five years we also had two record wet winters during which there was severe flooding. This is a sign of the fundamental and profound way that Britain’s climate is changing.”

He said the weather change could also have disastrous consequences for wildlife. “If we have a cold [snap again] now there is a danger it will kill a lot of wildlife that has emerged early due to the spring-like temperatures.”

The spell of warm weather is expected to continue over the weekend, and bookmakers are offering odds of 5-2 that this summer will be the hottest on record.