They also used more than 112 million pages of A4 paper. According to Reuters calculations, that is the equivalent of more than 10,000 trees.

With M&A volumes grinding to a halt as a result of the credit crisis and banks trying to keep a lid on travel expenses, the impact on the environment is bound to be less severe this year.

The research was commissioned by Merrill DataSite, which provides password-protected websites that can store documents used for due diligence and says it is enjoying booming business as companies seek to cut costs. "We have seen a significant increase in the number of deals being conducted through our VDRs (virtual data rooms) in Europe over the past 12 months," said director Merlin Piscitelli.

The data referred to the past year, when European M&A advisers spent more than 110 million pounds (US$220 million) on flights and hotels carrying out due diligence on cross-border deals.

An average such deal involves five international flights per individual and 20,000 pages of paper.

Making checks on deals online can also save time, Merrill DataSite said. Half of the respondents that had used VDRs said using one cut the time required for due diligence by a third.

Researchers polled around 300 M&A advisers in the UK, Germany, France, Sweden and the Netherlands from the end of May through to mid-June. (Editing by Will Waterman)