The gang pulled onto the forecourt of Hockmeyers garage at Holdingham in a stolen seven-and-a-half ton lorry just after 2am on Monday morning.

The thieves came with an Iveco lorry stolen from the Doncaster area on stolen plates and carrying five 10,000-litre containers.

They broke open the garage's diesel storage tank, hooked it up to the containers and were in the process of pumping the fuel out when Graham Stevens, one of the Sleaford Business Improvement District security guards, disturbed them while on his patrol and they fled. The thieves had another vehicle stolen from the Harrogate area.

Christian Slingsby of Hockmeyers Garage said: "We discovered it because Graham the security guard from the BID was doing his patrols and called us at home.

"If he had not come along it would have been a hell of a lot worse. It is just an absolute nightmare."

The raiders left the pump running and diesel spilling across the forecourt, Lincoln Road and down storm drains into the neighbouring Beck. As much as 4,500 litres of diesel was drained from the tank before it could be re-sealed and up to 2,000 litres of that could have been spilled.

Police and firecrews were joined by Environment Agency staff to clean up, assisted by highways staff who blocked the road, causing major traffic congestion for the rest of the day with traffic queued back down the A17 as far as Heckington.

Absorbent bags, booms and barriers were put down on the road, forecourt and in the Beck to contain and soak up the spillage and the road was reopened by 8.30am yesterday (Tuesday).

Environment Agency spokesman Catherine Burbage said: "We are hoping there will be limited or very little environmental impact at the moment."