BP, the UK energy group, has discovered a “giant” oilfield in the Gulf of Mexico that shows a new frontier opening up for US oil production.

The Tiber field, in more than 1,200m of water about 250 miles south-east of Houston, is one of the biggest discoveries in recent years. It is thought to hold at least 3bn barrels of oil – of which about 500m would be recoverable with today’s technology – and could be significantly larger.

The field was found by the deepest oil well ever drilled, reaching more than 9.4km – almost six miles – below the sea bed.

BP is pioneering the exploration of the Lower Tertiary or Paleogene area of the Gulf of Mexico, which lies deeper than the strata that provide most of the region’s known reserves.

The Tiber discovery, following the Kaskida field found in 2006 – which is also believed to hold 3bn-plus barrels of oil – is further evidence that there are large volumes yet to be found in the Gulf.

BP believes there could be a further 20bn barrels or more in the deep water, on top of the US’s proved reserves of about 30bn barrels,

Andy Inglis, BP’s head of exploration and production, told the Financial Times that Tiber “points to the longevity of the Gulf of Mexico as an area with significant productive potential”.

BP is already the largest oil and gas producer in the Gulf. Tiber and Kaskida could lift its output from 400,000 barrels of oil equivalent a day to about 650,000 boe/d.

The two fields could add at least 5 per cent to BP’s proved reserves, which were 18.1bn boe/d at the end of last year.

Production in the US is highly valued by oil companies as the fiscal and regulatory framework is more attractive than in most resource-rich countries.

However, BP warned that there was more work to be done to confirm the size of the field.

Production at extreme depths, at very high pressures and temperatures, is extremely challenging, and the field is unlikely to be on stream until the second half of the next decade.

BP’s shares were up 4.3 per cent at 541.7p, making them the biggest gainers on the FTSE 100 index, which was flat on the day.