A study by Reading University found that city offices are also now heavily leveraged and are increasingly owned by private equity vehicles, many with short-term interests. The proportion of overseas ownership has more than doubled in less than 10 years, from 20% to 45%, or 36 million sf of office. Multiple owners now account for 54% of the total floor space available in the city. The debt outstanding on all commercial property in this country stood at £138 billion at the end of 2005, more than three times the size of the lending peak that preceded the last property crash.

"Traditionally, city property owners took a long-term view on their investments and traded infrequently, with almost all offices having a single owner," the study stated. "By contrast, much of today's investment activity seems focused on short-term gains with less regard for risk. This, in turn, raises a key question: What if a substantial number of investors exited the market at the same speed as they entered?"

The sheer weight of money chasing city investments has pushed up the value of property. Between January 2003 and December 2005, more than euro 20.4 billion ($26.13 billion) was invested, 45% of which came from overseas investors.