BRITISH Airways (BA) plans to invest GBP150 million (about HK$1.71 billion) expanding its cargo facilities at London's Heathrow airport in a joint development with BAA, the site operator. BA said the project will be the airline's largest single investment in an airport construction venture and will more than double its cargo handling capacity at Heahtrow by the year 2000. The BA and BAA boards have approved the project in principle. Final approval is expected by the end of next year. The scheme will involve three phases which, when completed, will increase BA's Heathrow cargo handling capacity from 451,000 tonnes a year to around one million tonnes a year in 2000. Construction of the first phase is expected to start by the end of 1994 and should be completed by late 1996. The second and final phases are expected to be ready in 1998 and 2000. Details of the development and funding of the new cargo terminal are still being worked out. BAA also regards the schemes as an important part of its efforts to boost freight activities at Heathrow, the world's biggest international passenger airport. Mr Kevin Hatton, managing director of BA's world cargo division, said the airline intended ''to take advantage of the latest, proven technology to achieve maximum automation of handling functions''.