Britain's farmers warn the extreme summer has been 'completely brutal' with 2026 set to be the worst harvest on record – and a sixth heatwave of the year on the way.The exceptionally hot and dry spring and summer could have cut the country's cereals and oilseed harvest by 2.5million tonnes compared to earlier forecasts.This shortfall could be costing UK farmers as much as £390million, according to concerning analysis by the Energy and Climate Intelligence Unit (ECIU) think tank.Forecasts in March and April from European and US agricultural bodies suggested that UK yields were on course to be at or above ten-year averages in 2026.But fresh harvest data from the Agriculture and Horticulture Development Board suggests wheat, barley and oat yields will all be down. Wheat, around half of which has been harvested, will average 6.8 tonnes per hectare, well below the average.If the provisional yield estimates hold, the UK cereal and oilseeds harvest would be 19.5million tonnes - overtaking 2020 as the worst on record since detailed records began in 1984, even after good yields for oilseed rape, the ECIU warned.Even if things pick up, 2026 is likely to be one of the five worst harvests on record – four of which have occurred this decade amid increasingly extreme weather.The UK has already had five heatwaves across May, June and July – and forecast models are now suggesting a sixth heatwave of the year could begin this weekend.Before then, thundery showers are set to batter the North and Scotland - but central and southern England are likely to miss out on most of the much-needed rain. A tractor is shrouded in dust as a farmer tows a disc harrow across a dry field in Kent this week