There could be up to 100 billion barrels of oil in the South of England, according to exploration firm UK Oil and Gas Investments (UKOG).
The company drilled a well close to Gatwick Airport in 2014 and analysis has showed the local area could contain around 158 million barrels per square mile.
UKOG said the discovery was one of the most significant in the past 30 years and was a "world class potential resource".
"We think we've found a very significant discovery here, probably the largest [onshore in the UK] in the last 30 years, and we think it has national significance," the firm's chief executive Stephen Sanderson told the BBC.
Only a fraction of the total discovery would be extractable, the group said, owing to the geology in the Weald region, which spans parts of Sussex, Kent, Surrey and Hampshire.
"Based on what we've found here, we're looking at between 50 and 100 billion barrels of oil in place in the ground," says Mr Sanderson.
"We believe we can recover between 5% and 15% of the oil in the ground, which by 2030 could mean that we produce 10%-to-30% of the UK's oil demand from within the Weald area."
The company would need to continue drilling and testing to prove the initial results.
Many industry experts have predicted that fracking would be needed to extract commercial quantities of oil from the South of England.
UKOG, which has previously said it does not intend to use fracking, said the oil it has discovered is naturally fractured, giving it "strong encouragement that these reservoirs can be successfully produced using conventional horizontal drilling and completion techniques."
The South of England is already an oil producing region but the UK's main production sites are offshore in the North Sea. The offshore sites have produced around 45 billion barrels of oil over 40 years. Production levels have plummeted in recent years and major oil companies have announced plans to cut staff numbers at North Sea projects.