Experts at agricultural insurer NFU Mutual said they had seen more thefts in all areas of the North East, with Northumberland farmers in particular suffering.
In one incident last month in the Alnwick area, 1,200 gallons of the fuel – which can be used legally only in farm vehicles – was stolen the day after it had been bought for about £2,700.
There has also been a higher-than-average number of thefts reported in County Durham.
Industry watchers say the fuel is being stolen from vulnerable tanks on farmland.
Much of it is then sold to haulage firms, which have seen costs soar as the price of ordinary white diesel hits all-time highs. But some may also be used by ordinary motorists.
A spokesman for NFU Mutual said: "We believe most of the increase is down to gangs who move around the region at speed.
"But I'm sure some is also being stolen by ordinary people in diesel cars, who just drive off after filling their tank.
"With the price of normal fuel so high, it is very tempting. Nationally we have seen a 30% increase in red diesel theft and that is reflected in the North East.
"It is adding to feelings of fear amongst the rural community."
And the impact of the theft is not just financial – it can also cause serious environmental damage.
Ken Johnston, of NFU Mutual at Alnwick, said: "On one occasion the robber used a hacksaw to cut off the valve of a tank.
"He took what he wanted from the tank but then obviously couldn't block the hole. The rest of the diesel just drained away. This has happened a few times and it causes huge environmental damage, not just to the field but also to the watercourses it drains into.
"The costs of cleaning up the mess are higher than the value of the diesel.
