There could be up to 100 billion barrels of oil onshore beneath the South of England, says exploration firm UK Oil & Gas Investments (UKOG).
Last year, the firm drilled a well at Horse Hill, near Gatwick airport, and analysis of that well suggests the local area could hold 158 million barrels of oil per square mile.
But only a fraction of the 100 billion total would be recovered, UKOG admits.
The North Sea has produced about 45 billion barrels in 40 years.
“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,” Stephen Sanderson, UKOG’s chief executive told the BBC.
UKOG says that the majority of the oil lies within the Upper Jurassic Kimmeridge formation at a depth of between 2,500ft (762m) and 3,000ft (914m).
Most experts believe fracking, or hydraulic fracturing, will be needed to get commercial quantities of oil from the region.
Concerns over fracking led to large-scale protests when Cuadrilla drilled at Balcombe, West Sussex, in 2013.