Details of the find are likely to be given by John Browne, chairman of BP's exploration division, at a meeting with analysts on Thursday.
It is the first important briefing by Mr Browne for two years and has already kindled much excitement in the oil industry. He is also expected to elaborate on BP's two other big projects, in Colombia and the west of the Shetlands.
The Vietnamese find is thought to contain between two and three trillion cubic feet of gas - equal to about 500 million barrels of oil, which is equivalent to a substantial North Sea find such as the Nelson field.
The company is expected to press ahead with the early development of the field primarily for the burgeoning Vietnamese market, although there are hopes that the gas could be exported to Thailand in the long term if relations between Hanoi and Bangkok improve.
Oil industry experts say the project could also open up opportunities for several other big UK companies in Vietnam. BP is talking to British Gas about ways of developing the local gas market and related infrastructure. One issue under discussion is the construction of a gas-fired power station - a project that is also being examined by Britain's two electricity generators, PowerGen and National Power. Both generators say that they are considering a move into Vietnam, but it is too early to talk of any specific projects there.
Vietnam is widely regarded as one of the world's most promising oil regions. BP's find is understood to have been made in block 6 - one of two blocks off the country's southern shores in which the company has a 45 per cent interest. The blocks were acquired with Statoil, the Norwegian state oil company, and cover an area of about 10,000 sq km.
BP also owns a two-thirds interest in a 2,500 sq km adjacent block, 5-2, and a share in four other Vietnamese offshore blocks totalling about 16,000 sq km.
Mr Browne is also expected to provide further details of BP's plans for its huge oil and gas discoveries in Colombia, where it has discovered three oil and gas fields with total reserves equivalent to at least 5 billion barrels of oil.
Analysts believe news of the Vietnamese plans could boost BP shares futher. They slipped 2p to 418p on Friday.
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