Farmers have been urged to improve the security of their fuel supplies as the cost of red diesel thefts increased by 30% in the first five months of the year.

More than 50 claims for stolen red diesel, totalling more than £100,000, have been made to rural insurers NFU Mutual so far this year.

The claim value is almost a third higher than the same period last year, it said.

“It’s not just the cost of the diesel that’s the problem,” NFU Mutual spokesman Tim Price said.

“Very often thieves take the diesel and leave the tank open so it runs away and causes major pollution, which costs more to clean up than the diesel costs.”

Theft of domestic heating oil was also becoming a problem on farms, Mr Price said.

Precautions such as installing gates and floodlighting and resiting tanks behind buildings would help deter thieves, he added.

“Red diesel value is continuing to rise and heating oil is liquid gold at 60p/litre. It really is at serious risk of being stolen.”

Arable and dairy farmer Eddie Cowpe said he faced costs of up to £70,000 after thieves raided his 10,000-litre diesel tank and allowed fuel to spill onto his 142ha (350 acre) farm in Samlesbury, Lancashire last week.

“They broke the padlock to the yard, took what they wanted and let the rest go to waste. It’s very annoying,” he said.

“We have had to dig the yard out, intercept field drains and get a company to mop up the local brook. We think it will cost us about £70,000 to clean up.”

Mr Cowpe said he has now put up a secure fence around the tank in a bid to deter thieves.

Richard Martin, Environment Agency pollution policy adviser, said it was “good practice” to make fuel secure from both theft and accidental spillages to avoid enormous clean-up costs.