Chips were an inch shorter after last summer's heatwave damaged potato harvests – and more staple British foods could be threatened in future by extreme weather driven by climate change, campaigners claim.

Yields of fruit and vegetables, from the humble spud to grapes producing British wine, could be hit by extremes such as longer-lasting and more intense heatwaves, downpours and flooding, according to a report by the Climate Coalition of environmental groups.

It says last year's heatwave and drought – which Met Office analysts said was made 30 times more likely by climate change – saw yields of potatoes fall 20pc compared to the previous season, while growers also reported carrot yields were down by 25-30pc, and onions down by 40pc.

As well as a drop in potato yields, farmers had problems with smaller and misshapen spuds – leaving the average British chip more than an inch smaller, the report says.

And drought is not the only climate concern, as more than half of UK farmers say they have been affected by severe flooding or storms in the past decade, warns the report.

The coalition claims shoppers could find British-grown potatoes and other fruit and vegetables harder to come by in the future as a result of the changing climate.

Norfolk farmer Tony Bambridge is managing director of potato specialists B&C Farming at Marsham. As a result of last year's poor harvest, he told the EDP in October that his customer McCain had reduced its specification by 5mm to allow smaller potatoes to be processed into chips.

To prevent this becoming a regular occurrence, he said a more 'strategic approach' to water management was needed, including more financial support for farmers to build reservoirs, and longer-term abstraction licences so they can guarantee they will be able to fill them during the winters.

'Our irrigated crops actually performed very well last year – it was the unirrigated crops which had a problem,' he said. 'Climate change is challenging us as farmers to put in measures to be more resilient, which in this instance is coping with less water and irrigation, but equally if you go back to 2012 we also need to look at how we cope with having too much water as well.

'As a farming industry trying to feed the nation, we have to look at how resilient our soils and systems are to both extremes.

'We need more support for the industry to build reservoirs. If you see food as a 'public good' then one could argue that money taken away from direct payments [the EU subsidy system due to be phased out after Brexit] could be redirected in aiding farmers to build reservoirs. But they are a long-term project and they may take 20-30 years to pay back that investment.

'Some parts of Norfolk and East Anglia have less rainfall than Jerusalem, and that is considered an arid desert. This has significant challenges for the environment and public water supply, but more often than not it is agriculture that will lose licences because of how the system works.

'Currently the Environment Agency will only give abstraction licences for six years so it only gives surety that you can use that reservoir for six years, rather than 20-30 years to ensure you get that a return on the investment.

'That is why BAWAG (Broadland Agricultural Water Abstractors Group) and the NFU (National Farmers' Union) are very keen on looking at a 25-year water plan for the region. We need to think much more strategically about how we manage our water in the future.'

The Climate Coalition – which is made up of 130 organisations ranging from WWF, the Women's Institute, RSPB and National Trust, to aid agencies such as Christian Aid and Cafod – is calling for cuts to emissions driving up temperatures.

Gareth Redmond-King, from WWF, said: 'It should be unthinkable to us that the humble spud, a much-loved staple of the British diet, could become a delicacy.

'But the unthinkable becomes reality if climate change isn't tackled. To be able to enjoy our mash, chips or jackets for years to come, we need to take measures to tackle climate change urgently.'