* World's biggest 800 firms ranked on emissions transparency
* Only 37 pct publicly disclose complete emissions data
LONDON, May 1 (Reuters) - Most of the world's largest companies do not report their greenhouse gas emissions fully or correctly and do not have the data independently verified, a study by an environmental research body showed on Wednesday.
Companies are under pressure worldwide from policymakers, and a public increasingly concerned with green issues, to report the environmental fallout of all activities related to their daily business - from plane journeys to office supplies.
Officials hope the data generated can point to potential energy savings and encourage firms to reduce their emissions, while many companies see it as a way of planning for exposure to long-term costs such as taxes on emissions.
But for now London is the only stock exchange that forces all major companies to report in detail and many, particularly in emerging markets like Russia and across Southeast Asia, have all but ignored the idea.
The Environmental Investment Organisation (EIO) found that just 37 percent of the world's 800 largest companies companies disclosed complete data and correctly adopted the basic principles of emissions reporting.
Only 21 percent had their data externally verified and only one firm, German chemicals producer BASF, reported emissions across its entire value chain - from sources such as business travel, transport, distribution and investments. This transparency placed it at number one in the rankings.
"This ought to be a wakeup call for companies. Since the majority of total corporate emissions often come from (value chain) sources, large quantities of emissions are not being accounted for," said Sam Gill, chief executive of the EIO.
"Not only could this be a source of unmeasured risk for companies but it also means we are not getting the full picture in terms of corporate emissions," he added.
Companies are increasingly measuring and disclosing their environmental performance in their annual reports. However, the lack of a universally accepted or mandatory standard means both reporting formats and content vary widely.
The rest of the best 10 companies at reporting emissions were telecoms firms such as Canada's BCE, Singapore Telecom, Spain's Telefonica, BT Group and Deutsche Telekom, according to the EIO.
The bottom 10, with no publicly disclosed emissions data, was made up of mainly Russian and U.S. utilities and oil and gas companies, such as Phillips 66, Lukoil, Edison International and First Energy.
The EIO based its findings on the latest publicly available data, which for most companies was from 2011.