Insurers are under pressure because the scale of large companies is creating a massive concentration of risk, according to Aon Captive Services.

The division's chief executive, Stephen Cross, gave the warning at its annual client conference held in Athens.

Cross said the world's 10 largest companies now have total revenues of $2 trillion - more than the GDP of Germany - but that there is a lack of stability. "Over the past 20 years only 135 companies have maintained a presence in the US Fortune 500. It's an eye opener to see which household names have simply disappeared, merged, been bought, or are in Chapter 11," he told delegates.

In addition to commercial risk, the conference reported that companies are also facing greater exposure and potential losses from natural catastrophes.

Following last December's tsunami and Hurricane Katrina, Cross warned that scientists believe that the US coastlines are not immune from tidal waves. The potential values at risk are $3 trillion per coastline, which are not fully insured, he said.