Up to 150,000 litres of fuel leaked into farmland in Marlborough after thieves drilled through a major Esso pipeline.
The attack on the pipeline running through the estate of the late multi-millionaire Robert Sangster in Manton happened on April 2, causing a high risk of an explosion.
News of the incident, in which a tapping device was used to break into the pipeline 8ft underground, has only just emerged. It pumps fuel from the Fawley Refinery in Hampshire to a distribution terminal in Birmingham.
A hose had been connected to the pipe, which carries a range of fuels, and was then hidden with soil. It was disturbed by a farmer, resulting in the leak.
Esso does not yet know how much fuel has leaked but says 150,000 litres would be the worst case scenario.
A spokesman said: “The pipeline was quickly repaired and has resumed operation and there is no indication of any impact on human health as a result of the leak.
“We remain committed to resolving the situation fully. At the moment we are trying to determine whether there has been a leak and if there has, where the fuel has gone.”
A 500-metre safety cordon was put into place while the scale of the incident was established and people living nearby moved as a precaution.
Wiltshire Fire and Rescue Service sent fire engines from Marlborough, Calne and Devizes, together with an operational support unit and an incident command vehicle from Swindon.
A fire crew remained at the scene for 32 hours as a precaution.
Police and the fire service said this week that they didn’t release details of the incident because they were acting on behalf of the Wiltshire and Swindon Local Resilience Forum, a body including fire, police and ambulance, Wiltshire Council and Public Health England, and it was up to the forum to make the incident public.
Town councillors were sent an email marked confidential by town clerk Shelley Parker giving brief details of the incident days afterwards.
At a full council meeting, on Monday, Carl Barber, the Marlborough fire crew manager, told councillors: “It’s extremely dangerous. That is an extremely pressurised line at times and it can be a danger.
“It was brought to our attention by the police as some was seen to be leaking and it did take 32 hours and seven appliances because our attendance was required while they isolated and dealt with the issue.”
Esso has depressurised the pipeline to limit leaks. It says there is still a quantity of diesel and petrol mixture in a hole approximately 1.4 metres deep on the Manton estate.
The firm contacted Action for the River Kennet to identify suitable monitoring points along the river in case of pollution.
The Environment Agency is working with Public Health England to evaluate the impact on water courses.
An Environment Agency spokesman said: “At present there is no impact to the River Kennet, but we continue to monitor the situation closely.”
Police in Hampshire are investigating a large quantity of diesel found in a large industrial unit in East Wellow on April 17.
It is believed that the tanks found were being filled from a sophisticated system which had tapped into a main fuel line.
Two men, aged 32 and 34, from the Salisbury area, were arrested on suspicion of conspiring to steal fuel. They are on bail pending further inquiries.
Detectives from Lyndhurst CID in Hampshire are working with other police forces, including Wiltshire, as part of a wider investigation into breaches of fuel lines in the south of England.