Christian Aid reports that official greenhouse gas figures hugely underestimate Britain's contribution to climate change.
Adding in emissions from UK-funded operations in other countries would raise the UK's share of the global total from 2% to about 15%, the charity states.
The charity is calling on the government to ensure that companies measure their emissions thoroughly. British companies wanted globalisation, it says, and must take responsibility for the associated emissions.
"Our research reveals a truly staggering quantity of unreported carbon dioxide is emitted around the world by the top 100 companies on the London Stock Exchange," said Christian Aid's senior climate change analyst, Andrew Pendleton.
"The government should now oblige companies to report their emissions properly," he told the BBC News website. "In our view, this is a litmus test of how serious they are about climate change."
Working with the environmental research company Trucost, Christian Aid attempted to calculate emissions associated with FTSE-100 companies.
"While only 2.13% of the world's CO2 emissions emanate from the UK's domestic economy," says the report, entitled Coming Clean, "through the process of globalisation, CO2 is emitted around the world on Britain's behalf, in China, India, Africa and elsewhere.
"Britain's apparently light carbon footprint rapidly begins to assume a much greater profile when worldwide investments made with British money, through the mighty City of London, are taken into account."
Christian Aid's argument is that Britain benefits from those investments, either by bringing cheap goods to the UK or by creating profits which flow back into the British economy, so the emissions ought at least to be accounted for within Britain.
Therefore, the UK would account for between 12% and 15% of the global total, according to this methodology.
