An attempt by the reclusive billionaire heir to the Mars chocolate bar fortune to ban drilling for natural gas on his Montana ranch was defeated in a court hearing late on Tuesday.

A judge ruled that Forrest Mars Jr, the former chief executive of the confectionary giant and the son of Forrest Mars Sr, the man credited with inventing M&Ms and the Mars bar, could not prevent Pinnacle Gas Resources from drilling on a 10,000-acre lease it holds for the land beneath Mars's Diamond Cross cattle ranch in south-eastern Montana.

On Monday, Mars employees at the ranch prevented Pinnacle from entering the property when it tried to force its way on to the ranch. Montana law gives companies the right to drill on private land provided they hold a valid lease. The lease expires tomorrow if the company does not commence exploration. Pinnacle said it expected to begin drilling before the end of the week.

"Who the surface owner is should not make any difference, and it didn't today," a Pinnacle attorney, Bryan Wilson, said after the ruling.

In the Diamond Cross case, Pinnacle Gas Resources and another company, Fidelity, have gas and oil leases on the ranch that predate Mars' ownership, according to public records and company officials.

Mars, who is worth an estimated $14bn, owns 82,000 acres along the Tongue river near Montana's border with Wyoming. He is opposed to the drilling because large amounts of underground water are pumped out to access the natural gas. Many farmers view that water as a precious reserve, given the extended drought in the western US.

Mars began buying land in south-eastern Montana in 2003, at the same time that natural gas exploration in the area was booming. He also has residences in Wyoming and Virginia. Since then he has joined several court actions challenging the natural gas industry.

An attorney for Mars, Loren O'Toole, said the objective was not to prevent exploration but to ensure that the water could be returned. "The point is, we can't lose all that water and at the same time have no provision to put it back," O'Toole said.

"Forrest has a lot of money, but he's in the same boat as anybody else," Beth Kaeding, chairwoman of the Northern Plains Resource Council, told the Associated Press news agency.

"If you don't own the mineral rights, it doesn't matter how huge your ranch is, how politically powerful you are, how much money you have. Mineral rights trump surface rights."

Forrest Mars Jr, who was not in court for the hearing, is in his 70s and is one of three children of Forrest Mars Sr, the son of the founder of the company.

Mars is one of the largest family-owned companies in the US, making confectionery, pet food and other products, from Snickers to Whiskas cat food, with $21bn in annual worldwide revenues and an estimated 40,000 employees. The three Mars siblings all featured in last year's top 20 list of the richest Americans, compiled by Forbes magazine.

Pinnacle sued Mars in December after the gas company's employees were told that they would be treated as trespassers if they attempted to enter the ranch.

In separate cases, Mars is suing Pinnacle over plans to develop coal-bed methane on land near his ranch, and is also seeking to block a new railway line in south-eastern Montana that will facilitate the exploitation of the area's coal reserves.