The oil giant has been carrying out a massive drilling programme costing millions of pounds in the region since last year. News of a big gas strike by BP has been trickling out from the Far East for months, but this is the first time an estimate of its size has emerged from within the group.
Although a full assessment of the find's commercial potential is likely to take months, sources suggest it contains the equivalent of 500 million barrels of oil - about the size of a large new North Sea field.
But the company is still some way from establishing the oil or liquids content of the field - details that could be the key to the area's development. However, PetroVietnam, the state oil company, is understood to be pushing hard for its early development to boost economic growth and generate foreign currency.
Both the company and Hanoi remain tight-lipped about the plans partly because of fears that it could rekindle some long-running territorial disputes with mainland China.
The find is another triumph for BP, which has chalked up several impressive successes in the past two years. In Colombia, the company found the giant Cusiana/Cupiagua fields with estimated reserves of 3 billion barrels.
Earlier this year, it also struck a big oil field off the west coast of the Shetlands, with initial estimates of about 500 million barrels. It was the biggest find in the British sector of the North Sea for five years and opens up an entirely new oil province.
Vietnam, widely regarded as one of the world's most promising oil regions, is part of BP's strategy to explore in frontier areas. It is the first western oil company to make a significant discovery there and many of BP's American rivals fear it will result in BP being granted further exploration acreage there. At present US companies are prevented under US laws from trading with Vietnam.
Hopes concerning the gas find were first raised by a 'blow-out' last spring after BP struck gas at very high pressures. Since then more wells have been drilled to ascertain the presence of some large gas and oil-bearing structures. The gas find is understood to have been made in block 6 - one of two blocks in the Nam Con Son (Saigon) basin, in which BP has a 45 per cent stake. The blocks were acquired with Statoil, the Norwegian state oil company, from ONGC, its Indian counterpart, last November and cover an area of 10,000 square kilometres.
BP also owns a two-thirds stake in a 2,500 sq km adjacent block, 5-2, giving it easy access to explore a vast area with good oil prospects. In addition, BP and Statoil hold exploration licences for another four offshore blocks, totalling about 16,000 square kilometres, in Vietnam.
Despite the encouraging news, further drilling work is continuing and at least four more wells are likely to be drilled in the next 12 months.
Oil analysts contacted over the weekend said the gas find could provide an added fillip for BP's share price, which closed at 305.5p last Friday. Earlier this month, the group announced better than expected net profits of pounds 507m for the half-year to 30 June, against a pounds 711m loss last year.
Iraq yesterday accused Saudi Arabia and Kuwait of breaking production quotas set by Opec, the oil exporters' cartel, and flooding the oil market, Reuter reports.
The attack, carried in the ruling Baath party newspaper, Thawra, comes as Baghdad is stalling on a United Nations offer allowing it to sell oil worth dollars 1.6bn to pay for humanitarian supplies and financial obligations set under terms of the Gulf war ceasefire.
'It is widely known that the oil policy pursued by al-Saud (the Saudi ruling family) focuses on flooding the world markets with extra oil . . . for the service of the US,' it said.