By Tom Bawden / Independent

Farmers will be on high-alert this weekend as the heatwave threatens to unleash a wave of fires that could ruin crops across the country, a senior Met Office scientist has warned.

Update, Fri 19 July 18:00 BST: Spike in A&E admissions, wildfires and melting roads as Britain battles seventh day of 30-plus temperatures

As Britain clocked up its sixth consecutive day of plus-30C temperatures – with the thermometer peaking at 30.4C – soon-to-be harvested crops such as wheat and winter barley are looking particularly vulnerable to fire, said Karl Kitchen, the Met Office scientist with responsibility for wildfires. He issued his crop-fire warning as the country continued to reel from the longest heatwave for seven years.

In Consett, County Durham, a 21-year old father died after falling from a flat roof while sunbathing, while it emerged that Graham Bennett, a postman of 29 years, collapsed and died on Monday while doing his round on the Ermine estate in Lincoln.

As many as 760 people are thought to have died so far as a result of the heatwave, as the death toll of swimmers drowning as the sun enticed people into Britain’s dangerous open water sites hit at least 13.

In London, where there have been an average of 21 grass fires a day this month, an area the size of four football pitches caught fire on Mitcham Common near Croydon. Meanwhile, donations of O and B blood groups are down 11 per cent, the NHS said.

The temperature is forecast to dip slightly today, to a maximum of 29.0C. But the Met Office has today elevated the wildfire warning system it introduced this year from yellow to amber, meaning any grassfire will be extremely difficult to contain.

The Met Office this morning issued a Level 3 heatwave warning for the north-west of England, but reduced London and South East’s to Level 2.

Grass and “elevated” crops are particularly susceptible to fire and the risk posed by the prolonged hot, dry weather will accelerate this weekend as the wind picks up and humidity decreases. The Met Office said: “Anywhere south of Liverpool and Manchester and in Wales will be vulnerable.”

Wheat crops are said to be particularly vulnerable because they are “very aerated” and the lower the plant density the easier it is for the flames to spread. Guy Gagen, chief arable advisor to the National Farmers Union (NFU), said: “There is a risk at this time every year, but this year that risk is elevated. This is the driest and warmest period since 2006 and means grass, instead of growing, will die off and spread the risk.”

Mr Gagen said that Britain’s 300 square kilometres of winter barley crops would be most at risk over the weekend because they are ripe and in the process of being harvested. Moving closer to August, without substantial rainfall, the 2,000 square kilometre wheat crop will become most vulnerable as it begins to harvest, he said. In the case of both crops, the danger is spread across the UK but greatest in East Anglia where a large portion of the plants are grown. Ripe crops are most flammable with crops less likely to burn when they are greener.

One person enjoying the hot weather is the Deputy Prime Minister Nick Clegg, who today revealed that he is wandering around in his socks to keep cool in the office. “Obviously in public events when I have to respect the dignity of the office I put my shoes back on,” he told London’s LBC radio.

Texans warm to climate change

Americans’ belief in the existence of climate change is more closely linked to the thermometer than previously thought, a survey reveals. Following a winter of record snowfall in 2010, the US public’s acceptance of climate change fell to a low of 52 per cent. By March this year it had staged something of a recovery, rising to 65 per cent. In the July heat, climate change acceptance is now at 70 per cent, according to the poll by the University of Texas.