The possibility is growing that Britain in 2007 may experience a summer of unheard-of high temperatures, with the thermometer even reaching 40C, or 104F,a level never recorded in history.

The likelihood of such a "forty degree summer" is being underlined by the tumbling over the past year of a whole series of British temperature records, strongly suggesting that the British Isles have begun to experience a period of rapid, not to say alarming, warming. This would be quite outside all historical experience, but entirely consistent with predictions of climate change.

The Met Office's Hadley Centre for Climate Prediction and Research, in a joint forecast with the Climatic Research Unit of the University of East Anglia, has already suggested that 2007 will be the hottest year ever recorded globally.

Its long-term forecast for this summer in Britain is much more cautious, merely predicting that temperatures this year will be "above average". However, the suite of new records for the UK established in the past 12 months, culminating in an April of unprecedented high temperatures, is pointing to something new happening to the British climate.

The incredibly warm April days we have been experiencing are not just wonderful, they are downright weird when seen in their seasonal context. Some of them have been 10C hotter, or more, than they should be at this time of the year.

Average maximum temperatures at the end of April in southern England are traditionally about 13C or 14C. This weekend in London and the South-east, the thermometer may hit 26C or even 27C - 79F to 80F.

An air temperature of 80 in April seems to belong to fantasy land. In the childhood of anyone aged over 40, it was a rare enough temperature in August.

Even with its end not yet here, this month is certain to be the hottest April ever recorded. But that's just one of a cascade of British temperature records which are now falling.

Spring 2007 (defined as March, April and May) will probably be Britain's hottest spring. It has followed the second-warmest winter in the UK record (December, January and February) and the warmest-ever autumn (September, October and November 2006).

Before that, we had Britain's hottest-ever month (July last year), which included the hottest-ever July day (19 July, when the temperature at Wisley, Surrey, reached 36.5C, or 97.7F, beating a record that had lasted since 1911).

To crown it all, yesterday the Met Office announced that the past 12 months, taken together, have been the hottest 12 months ever to have occurred in Britain, with a provisional mean temperature of 10.4C. The previous record (March 1997 to April 1998) was 9.7C.

This leap of nearly three-quarters of a degree is huge and should make everybody consider whether a major shift in Britain's climate is becoming visible. To answer Yes to that question is by no means unreasonable.

It raises the possibility that in 2007 Britain may experience for the first time the sort of "extreme event" heatwave that supercomputer models of climate predict will hit Britain as global warming takes hold.

A heatwave of this nature hit northern and central France in the first two weeks of August 2003 and caused 18,000 excess deaths (part of a total of 35,000 excess deaths in a wider area including Switzerland, northern Italy and southern Germany). Many of the dead were old people with breathing difficulties who collapsed when night-time temperatures never dropped below the 80s Fahrenheit.

The temperatures recorded during this episode were so far above the statistical record that it is accepted by meteorological scientists as having been caused by climate change - and is regarded as one of its first manifestations in Europe.

Even though Britain was not at the centre of the heatwave, the UK temperature record was resoundingly smashed by it. On 10 August 2003, the 100F mark was breached for the first time ever, with a reading of 38.5C, or 101.3F, at Brogdale, near Faversham in Kent.

The previous record had been 37.1C, or 98.8F, set on 3 August 1990 at Cheltenham, Gloucestershire, and thus the jump was 1.4 degrees Centigrade or 2.5 degrees Fahrenheit, an absolutely enormous leap.

Despite the astonishing April, the natural variability of the climate is such that there is no guarantee whatsoever that the 2003 record will be broken this summer. But the indications are pointing that way. And if 2007 summer temperatures do go even higher, hitting the 40C/104F mark, there might well be severe problems for the public services, not just with drought and water shortages, but with large-scale heat exhaustion.

A side effect might well be to make it extremely hard for people who do not accept that climate change is happening to deny the reality of a warming world.

"The effects of temperature rise are being experienced on a global scale," Dr Debbie Hemming, a climate scientist at the Hadley Centre, said last night.

"Many of the regions that are projected to experience the largest climate changes are already vulnerable to environmental stress from resource shortages, rapid urbanisation, population rise and industrial development."

If you want to bet on the temperature exceeding the 100F mark this summer, Ladbrokes will only quote odds of 3-1.

The bookies aren't stupid. And they may well be right.

Overheating Britain

* The winter of 2006-2007 was the UK's second-hottest ever

* Autumn 2006 was the hottest ever

* July 2006 was Britain's hottest ever month

* Hottest ever 12-month period: 31 April 2006 to 1 May 2007 (provisional mean temperature: 10.4C)

* Previous hottest: 31 March 1997 to 1 April 1998 (9.7C)